"It's kind of like horse racing. If you're a horse that does well in the last sprint, you wait. But if you wait too long, there's going to be too much distance for you to catch up and you're not going to make it," said Steffen Schmidt, professor of political science at Iowa State University.

While supporters are impressed with Obama's signs of strength - his fund-raising prowess, the huge crowds at rallies, the Internet following - some are getting anxious that he hasn't risen in the polls.

"I am worried that he hasn't done more to close the gap," said a New York lawyer who has donated the maximum $2,300 to Obama's campaign and spoke on condition of anonymity. "It's positive that he hasn't fallen farther behind."

Preeta Bansal, a New York supporter, said she's not concerned.

"It's going to be a long fall. The race is just beginning and he's on fire," said Bansal, referring to recent stump speeches where she said he's been "superb."

New York City Councilman James Sanders, a Queens Democrat who supports Obama, conceded that supporters are "always concerned" and looking for the candidate to shine. But he said there's tremendous excitement for Obama, and real optimism that he can knock out Clinton.

"On the ground, the people, the groundswell is growing," Sanders said.

To overtake Clinton, the Obama campaign plans to drive home the message that the battle comes down to the following question: "Who do people think is most likely to bring about the change we need?"

David Axelrod, a top Obama adviser, said the Illinois senator won't hesitate to draw sharp contrasts with Clinton where contrasts are "germane and real." But don't expect a smackdown any time soon.

"I know there's a rooting interest, a kind of blood lust in the political community to see a kind of steel-cage match between Obama and Hillary," Axelrod said. "I don't think that's either politically smart or consistent with who he is."

The campaign has pinned much of its strategy on winning Iowa. Polls show Obama is in a close three-way race there with Clinton and John Edwards.

The campaign also has begun to deploy staff and build up ground operations in the states holding primaries on Feb. 5. And in the coming months, star supporter Oprah Winfrey "will do some things for us," Axelrod said.

"To me that's a continuation of the war," Edwards said. "Combat missions mean that the war is continuing. I believe the war needs to be brought to an end."

Edwards' broadside put Clinton on the defensive at a time when she is enjoying a comfortable lead in opinion polls and trying to appear above the fray. But there was no sign that the debate would prompt a major shake-up in the Democratic field.

"There may be a continuing counter-terrorism mission," she said, while adding that "the vast majority" of American troops would be out of Iraq by the end of her prospective first term in 2013.

The cross-fire on the campus of Dartmouth College came at the first debate of the critical autumn season leading up to the early voting contests in January ahead of the November 2008 election to replace President George W. Bush.

Iraq was a dominant theme of the debate as Democrats seek a way to force Bush to change his war strategy, which may leave as many as 100,000 troops there by the time the next president is inaugurated in January 2009.

He urged the U.N. to reform its Human Rights Council, which in the past has been chaired by Libya and other dictatorships, and said that the United States was open to an overhaul of the U.N. Security Council. The council is made up of the United States, Russia, China, Britain and France. Bush said he thought Japan was "well qualified" for membership.

The president began a three-day visit to New York on Monday for the 62rd meeting of the General Assembly. He met privately at the Waldorf-Astoria Hotel with Mahmoud Abbas, president of the Palestinian Authority, and Tony Blair, the former British prime minister who now represents the quartet -- the United States, Russia, the United Nations and the European Union -- seeking to bring about a broad peace agreement between the Palestinians and Israel.

The General Assembly speech today veered away from the themes of terrorism and war that were the foundation of Bush's first speeches at the U.N. Instead, he turned to elements of foreign policy that carry less of an edge while still encouraging the spread of democracy and the fight against tyranny.

The shift in tone comes at a time when Bush is struggling in a political world grown increasingly unfriendly, both at home and abroad.

It was at the U.N. last year that Venezuelan President Hugo Chavez -- never a friend of Bush -- likened the American president to a visitor from the underworld, saying, "The devil came here yesterday, and it smells of sulfur still today."

But even international allies are growing skittish. Britain, under new Prime Minister Gordon Brown, hopes to scale down its commitment of troops in Iraq. And at home, notwithstanding the Democrats' inability to force Bush's hand in Iraq, there are few signs that public opposition to the war is weakening.

The president's history of laying out a hard line and challenging the United Nations to join him frequently has left him searching for friends in an organization that has been described with scorn by the White House.

By contrast, the call for cooperation on a humanitarian agenda "certainly is a stronger message for a U.S. president than challenging the relevance of the United Nations," said P.J. Crowley, a senior fellow and the director for homeland security at the liberal Center for American Progress in Washington.

Johanns was expected to announce his candidacy as early as Monday for the seat being vacated by fellow Republican Chuck Hagel after two terms. Analysts say Johanns would be the front-runner of four men seeking the GOP nomination.

Nebraska is a Republican-leaning state. Democrats have talked of recruiting Bob Kerrey, a former U.S. senator and Nebraska governor who is now a university president in New York City. Democrats control the Senate, 51-49.

As agriculture secretary, the mild-spoken Johanns pressed U.S. trading partners to remove barriers to U.S. beef, erected out of fears of mad cow disease, and to expand farm exports, which account for a quarter of farm income. He took the lead in administration proposals to deny farm subsidies to the wealthiest Americans.

In his resignation letter to Bush, Johanns said the U.S. farm sector "is stronger than ever before," with high crop prices and record farm exports.

"After careful thought and difficult deliberation, I am writing to inform you that I have decided to pursue a new opportunity to serve this great nation," wrote Johanns.

Nebraska is a major grain and cattle-producing state. Democrats in Nebraska said Johanns was leaving USDA without completing an important task -- overhaul of U.S. farm policy this year.

Johanns resigned as governor to become agriculture secretary in January 2005. Born in Iowa, Johanns practiced law in western Nebraska before election as mayor of Lincoln, the state capital, in 1991 en route to the governorship in 1998. He once said that for a former farm boy, being agriculture secretary was a dream job.

Deputy Agriculture Secretary Charles Conner was named acting secretary until Bush nominates a permanent replacement.

The Agriculture Department, with 100,000 employees, is in charge of crop subsidies, the national forests, a vast research network and public nutrition programs including school lunch and food stamps.

Ferd Hoefner of the Sustainable Agriculture Coalition, said Johanns was the first USDA chief in decades to try to rein in farm payments and to show interest in beginning farmers.

But lawmakers have faulted Johanns for the faltering campaign to create a nationwide animal-tracking system, originally embraced by the administration as a key safeguard against mad cow and other fearsome diseases. USDA relies on voluntary participation in the program.

"He has been a clear and open advocate for ethanol within the administration," said Jay Truitt of the National Cattlemen's Beef Association. Truitt said Johanns "was the first to understand" how the explosive growth of the fuel ethanol industry would squeeze livestock and meat producers.

White House spokeswoman Dana Perino said the president hoped the US Senate, controlled by Bush's Democratic foes, would confirm Mukasey to replace the scandal-stained Alberto Gonzales before going into recess on October 8.

Gonzales, a close Bush confidant, left the post under a cloud, with Democratic and Republican critics alike charging that he lacked independence from the White House, was incompetent, hid the truth and may be guilty of perjury.

A former top aide to Gonzales revealed earlier this year that she improperly used political criteria in hiring decisions, and Democrats have been probing whether a mass purge of federal prosecutors was tied to political, not legal, considerations.

The nomination could have far-reaching implications for several pitched battles over Bush administration anti-terrorism policies, like warrantless spying on US citizens.

Democratic Majority Leader Harry Reid said Mukasey had "strong professional credentials a reputation for independence" and suggested the former judge "knows how to say no to the president when he oversteps the Constitution."

"But there should be no rush to judgment. The Senate Judiciary Committee must carefully examine Judge Mukasey's views on the complex legal challenges facing the nation," Reid said in a statement.

Democratic Senator Patrick Leahy, chairman of the Judiciary Committee that will take up the nomination before it goes to a vote, promised to review Mukasey's qualifications "a serious and deliberate fashion."

But in a potential hurdle to Mukasey, Leahy also warned that he would take into account White House "cooperation" on existing requests for information on issues like the mass firings and the warrantless surveillance program.

"The next attorney general needs to be someone who can begin the process of restoring the Department of Justice to its proper mission," Leahy, one of Gonzales's fiercest critics, said in a statement.

At the Rose Garden ceremony, which Gonzales did not attend, Mukasey thanked the embattled official for his "support and encouragement" but did not praise his record or his management of the Justice Department.

Mukasey said the department faced "vastly different" challenges from when he served there 35 years ago "but the principles that guide the department remain the same: To pursue justice by enforcing the law with unswerving fidelity to the Constitution."

Mukasey, 66, was appointed a federal district judge in New York under Republican former president Ronald Reagan and retired in 2006, returning to private practice.

He presided over several high-profile terrorism legal cases, including the trial of Omar Abdel-Rahman, the so-called "blind sheikh" who was convicted as the master of a 1993 attack on New York's World Trade Center towers.

Mukasey generally has been supportive of Bush administration policies in the war on terror, although he has at times ruled against the president.

He served as a district judge in the case of Jose Padilla, the US citizen accused by the Bush administration of being an enemy combatant who conspired to kill Americans overseas.

Mukasey upheld the Bush administration's right to detain Padilla indefinitely without charges -- a decision later reversed -- but ruled that he was entitled to a lawyer -- a position the administration argued against.

"That decision hardly makes Mukasey a wild-eyed civil libertarian," said Mark Agrast, a senior fellow with the left-of-center Center for American Progress think tank in Washington.

"But his insistence that the government give Padilla access to counsel was a rare act of principle at a time when few in Congress or the courts were willing to defy the administration," said Agrast.

Even if those proposals draw the 60 votes needed to overcome a Senate filibuster — a level that has eluded Democrats this year — any real strictures on the president would face a veto, frustrating war critics and raising the prospect that roughly as many American troops might be in Iraq a year from now as were there a year ago.

Still, the Democrats tried to get ahead of President Bush's planned speech on Iraq on Thursday night, and to press what they see as a political advantage in opposing the war in the months before the 2008 elections.

Senator Harry Reid of Nevada, the majority leader, and two party leaders on military issues accused Mr. Bush of embracing "more of the same" and of trying to pass off a routine troop reduction as a significant shift in policy.

"That is unacceptable to me, it is unacceptable to the American people," said Mr. Reid, who was flanked by Senators Carl Levin of Michigan, chairman of the Armed Services Committee, and Jack Reed of Rhode Island, a West Point graduate.

Democratic presidential contenders also assailed the administration's plan.

Senator Barack Obama of Illinois called for the withdrawal of one or two combat brigades a month, starting immediately.

Senator Hillary Rodham Clinton of New York said taking credit for the force reductions that Gen. David H. Petraeus, the commander in Iraq, was recommending, and that Mr. Bush appeared ready to accept, was "like taking credit for the sun coming up in the morning."

With Democrats intensifying their attacks on the strategy outlined this week by General Petraeus, the administration is setting in motion its plans with a prime-time speech by the president on Thursday, a subsequent visit to a military base and continued appearances by General Petraeus and Ryan C. Crocker, the American ambassador in Baghdad.

At a news conference on Wednesday, General Petraeus reiterated that he was unwilling to commit to troop cuts beyond a five-brigade reduction by mid-July, a level he described as prudent. There are 20 combat brigades in Iraq.

He also took issue with claims that such a reduction would not be significantly faster than what had already been scheduled. Combat forces in Iraq serve up to 15-month tours. Under that limit, part of the Pentagon's broad effort to lessen the strains on the military, General Petraeus would not have had to pull out any combat units until April, instead of removing the first brigades in mid-December, he said.

"We are coming out quicker than we had to," he said.

The White House spokesman, Tony Snow, in his last news briefing before leaving the job, rejected Democrats' complaints that the administration's plan was simply a return to the level that existed before more than 30,000 additional troops were sent into Iraq this year, a buildup that the administration pointedly referred to as a surge, suggesting its temporary nature.

"It's a different country," said Mr. Snow of Iraq. "You have the ability to reduce the numbers because there have been changes that reduced the necessity of American involvement."

Senator Reid would not provide details of the legislative proposals that Democrats will pursue. But Mr. Levin and Mr. Reed have been working with some Republicans on a measure that would focus the military mission on counterterrorism, training Iraqis and protecting forces already there — a switch intended to allow large numbers of combat troops to be withdrawn by next spring.

"We have to go ahead and recognize the strain on the military forces and give them the tasks that they can do so well," said Mr. Reed, a former Army captain, "but within the capability of their resources and the best interests of the United States."

They have been exploring the idea of making the withdrawal more of an objective than a requirement in order to attract Republican votes, but that approach could cause defections by Democrats.

Democrats have been picking up new Republican support for a measure that requires troops to spend at least the same amount of time at their home bases as they did in Iraq before returning — a requirement that could reduce troop numbers because the Pentagon would not have as many eligible for deployment.

"I think that might be a good way to accelerate a troop reduction," said Senator Gordon Smith, Republican of Oregon, who noted that it was also popular with strained military families.

That measure attracted 56 votes this summer, and some Republicans who opposed it then, including George V. Voinovich, Republican of Ohio, have expressed new interest. Senator Arlen Specter, Republican of Pennsylvania, said Wednesday that he was considering the proposal, and Democrats were also trying to persuade Senator Lisa Murkowski, Republican of Alaska.

Mr. Reid said Democrats also planned to vote on more aggressive legislative challenges to the war, which could help appease critics who are demanding that Democrats take tougher action.

Democrats say they may also be more willing to try to attach conditions to coming Pentagon spending requests. (Democrats have been reluctant to limit money for the war unilaterally.) "I think the American people are getting tired of sending the money with no end in sight," said Senator Charles E. Schumer, Democrat of New York.

The struggle to settle on a party alternative illustrates the problems Democrats are having finding a way to take on the president that unites their party and avoids criticism that they are weak on national security.

As Democrats huddled Wednesday to prepare for the floor debate, a group of leading House Republicans arrived in Iraq to demonstrate their backing for the president. The lawmakers, led by Representative John A. Boehner of Ohio, the Republican leader, had been in Iraq less than five hours, but in a conference call with reporters they said their initial briefings had already confirmed improvements.

"Clearly what's happened over the last three months has been real success," said Mr. Boehner, who previously visited Iraq in July 2006.

In an interview on "The Today Show" on Wednesday, Secretary of State Condoleezza Rice said stabilizing Iraq was part of "a long process of dealing with what the president called a long time ago a generational challenge to our security brought on by extremism, coming principally out of the Middle East."

Speaker of the House Nancy Pelosi, a California Democrat, said Ms. Rice's comment represented an acknowledgment that the United States would be engaged in Iraq for "years to come."

"We need a new direction that redeploys our troops from Iraq, rebuilds our military and refocuses on fighting terrorism across the world," Ms. Pelosi said.