The latest move in the separation of powers fight between the legislative and executive branches came as members of Congress began returning from their Fourth of July recess. An atmosphere of high tension accompanied the resumption of work as a fight also loomed there between majority Democrats and some key Republicans and Bush over his Iraq war policy.

In his letter regarding subpoenas the Judiciary panels issued, Fielding said, "The president feels compelled to assert executive privilege with respect to the testimony sought from Sara M. Taylor and Harriet E. Miers."

"You may be assured that the president's assertion here comports with prior practices in similar contexts, and that it has been appropriately documented," the letter said.

Fielding was responding to a 10 a.m. EDT deadline set by the Democratic chairmen, Sen. Patrick Leahy of Vermont and Rep. John Conyers of Michigan, for the White House to explain it's privilege claim, prove that the president personally invoked it and provide logs of which documents were being withheld.

As expected, Fielding refused to comply. He said he was acting at Bush's direction, and he complained that the committees had decided to enforce the subpoenas whether or not the White House complied.

"The committees have already prejudged the question, regardless of the production of any privilege log," Fielding wrote. "In such circumstances, we will not be undertaking such a project, even as a further accommodation."

Conyers' response left little doubt where the matter was headed.

"Contrary what the White House may believe, it is the Congress and the courts that will decide whether an invocation of Executive Privilege is valid, not the White House unilaterally," the House chairman said in a statement.

The privilege claim on testimony by former aides won't necessarily prevent them from testifying this week, as scheduled.

Leahy said that Taylor, Bush's former political director, may testify as scheduled before the Senate panel on Wednesday. The House Judiciary Committee scheduled Miers' testimony for Thursday, but it was unclear whether she would appear, according to congressional aides speaking on condition of anonymity because negotiations were under way.

The exchange Monday was the latest step in a slow-motion legal waltz between the White House and lawmakers toward eventual contempt-of-Congress citations. If neither side yielded in that circumstance, it would go to a federal court.

The probe into the U.S. attorney firings was only one of several Democratic-led investigations of the White House and its use of executive power spanning the war in Iraq, Bush's secretive wiretapping program and his commutation last week of I. Lewis "Scooter" Libby's prison sentence.

Meanwhile, several Democratic-run investigations are playing out this week as they head toward contempt of Congress citations and, if neither side yields, federal court.

On Iraq, Democrats expect to resume legislative challenges to Bush's policy on the war as the Senate this week takes up a major defense spending bill. The administration has been concerned about an escalation of Iraqi war fervor. So much so that Defense Secretary Robert Gates canceled a four-nation South American tour this week to work with the White House on Iraq policy. unclear whether she will appear.

Sen. Susan Collins of Maine, ranking Republican on the Homeland Security Committee, said Monday there had been "a steady erosion for the president's policy" in Congress because of the "tremendous loss of life among our troops" in June and "the failure of the Iraqi government to pursue the political reforms that are necessary to quell the sectarian violence."

Collins is among the Senate Republicans seeking to see U.S. troops departing Iraq by early 2008. Bush's strategy for a short-term troop increase to stabilize Baghdad and certain parts of Iraq has not been successful, she said.

"The president argued that we needed to undertake the surge in order to give the Iraq government the time, the space to pursue the political reforms," Collins said on CNN. "That hasn't happened. Instead it has been our troops who are making the sacrifice, who are bearing the burden, and that's why you see a real change in support for the Iraq strategy."

In Baghdad Monday, Iraqi Foreign Minister Hoshyar Zebari warned that a quick American troop withdrawal could lead to civil war and the collapse of the Iraqi state.

"This process has been drug-out a long time, which says to me it is political," he said. "There is no wrong doing."

Opposition Democrats are calling for the Attorney General's ouster over what they say is the politically-motivated firing of eight federal prosecutors. The Justice Department says those dismissals were based on poor performance.

During this trip to Europe, President Bush also suffered a set-back to his top legislative priority of the year: comprehensive immigration reform.

A bipartisan bill fell short of the votes it needed in the Senate because some members of the president's own party object to its provisions giving illegal immigrants a path to citizenship.

The president says he knows it is a tough debate, but he is still optimistic about getting the bill through.

At dusk, illegal immigrants prepare to use an inner-tube to cross the Rio Grande River at the U.S-Mexico border in Nuevo Laredo, Mexico, 02 June 2007
"I believe we can get an immigration bill," he said. "Now it is going to require leadership from the Democrat leaders in the Senate, and it is going to require me to stay engaged and work with Republicans who want a bill."

While in Europe, the president telephoned three Republican Senators to lobby for the immigration bill. He will go to Capitol Hill Tuesday to press them further.

"It is important that we address this issue now and I believe we can get it done," he said. "Listen, a lot of progress was made between people in both parties making hard decisions necessary to move a comprehensive plan."

Mr. Bush says the legislative process often takes two steps forward and one step back. He says he will start Tuesday to work toward taking some steps forward again, telling a reporter on the trip, "I will see you at the bill signing."

"The United States takes this issue seriously,"  Mr Bush said.

"The new initiative I'm outlining today will contribute to the important dialogue that will take place in Germany next week."

He promised that America would work with other countries "to establish a new framework for greenhouse gas emissions for when the Kyoto Protocol expires in 2012".

The President's language is a marked contrast to that of earlier in his administration when he questioned whether climate change was a man-made problem and refused to ratify the 1997 Kyoto Protocol, not least because it exempted China and India – who were emerging as competitors – from the first round of emissions cuts.

But his administration also made plain today that it remained opposed to Germany's call for the imposition of a strict upper limit, allowing global temperatures to increase by no more than 2C.

These would mean a worldwide reduction in emissions effectively of 50 per cent below 1990 levels by 2050.

Nor is Mr Bush willing to accept EU proposals for a global carbon-trading program under which companies would buy and sell pollution permits.

Paula Dobriansky, the US Under Secretary of State, told reporters today  that the administration doubted the effectiveness of measures that "would restrict economic growth and investment for new research".

She added: "There is not a one-size-fits-all or silver-bullet solution to climate change."

The US argues that its own progress in cutting carbon dioxide emissions in 2005 shows that a less rigid policy of promoting new technology can also achieve results.

Mr Bush said that all countries, including "rapidly growing economies like India and China" needed to "establish midterm management targets and programmes that reflect their own mix of energy sources and future energy needs".

His proposals did little to impress environmental groups.

Brent Blackwelder, president of Friends of the Earth, branded them a "a complete charade", adding: "It is an attempt to make the Bush administration look like it takes global warming seriously without actually doing anything to curb emissions."

There was a more muted reaction from European leaders such as British Prime Minister Tony Blair and Angela Merkel, the German Chancellor, who have led international pressure for more action on climate change.

Mr Blair, speaking in South Africa, said there was now a "real chance" of a deal on global warming at the G8 summit and then appealed for greater understanding over Mr Bush's position.

Mr Bush has also sought to dampen down growing tensions with Russian President Vladimir Putin, over the stationing of US interceptor missiles in Eastern Europe, inviting him to a meeting at the Bush's family holiday residence at Kennebunkport, Maine, next month.

Today, however, Mr Putin raised the rhetorical stakes further, saying that the recent test of a new Russian missile was a direct response to US actions and "imperialism" in world affairs.

"We are not the initiators of this new round of the arms race," he added.

Bush spoke at the nation's largest training center for law enforcement.

He chose the get-tough setting as conservative critics blast a Senate proposal as being soft on people who break the law. Hoping to blunt that message, Bush emphasized that any new options for immigrants and foreign workers would not start until tougher security is in place.

The presidential stop came during a congressional recess, with senators back home and facing pressure from the left and right on the immigration plan. Bush's aim is to build momentum for the legislation, perhaps his best chance for a signature victory in his second term. The Senate expects to resume debate on it next week.

"A lot of Americans are skeptical about immigration reform, primarily because they don't think the government can fix the problems," Bush said.

"And my answer to the skeptics is: give us a chance to fix the problems in a comprehensive way that enforces our border and treats people with decency and respect. Give us a chance to fix this problem. Don't try to kill this bill before it gets moving," Bush told students and instructors at the Federal Law Enforcement Training Center.

Bush repeatedly cast the matter as one of political courage.

"Those determined to find fault with this bill will always be able to look at a narrow slice of it and find something they don't like," the president said. "If you want to kill the bill, if you don't want to do what's right for America, you can pick one little aspect out of it.

"You can use it to frighten people," Bush said. "Or you can show leadership and solve this problem once and for all."

The bill would give temporary legal status to millions of unlawful immigrants, provided they came forward, paid a fine and underwent criminal background checks. To apply for a green card, they would have to pay another fine, learn English, return to their home country and wait in line.

The plan also would create a guest worker program. It would allow foreign laborers to come to the U.S. for temporary stints, yet with no guarantee they can eventually gain citizenship.

Both the new visa plan and the temporary worker program are contingent on other steps coming first. Those include fencing and barriers along the Mexico border, the hiring of more Border Patrol agents and the completion of an identification system to verify employees' legal status.

The legislation would also reshape future immigration decisions. A new point system would prioritize skills and education over family in deciding who can immigrate.

Georgia's senators both played leading roles in producing Bush's deal with the Senate. Yet they have also said they may not support the final bill, depending upon how it is amended.

Bush chastised those who say the proposal offers amnesty to illegal immigrants. He called it empty political rhetoric.

The U.S. House advanced a final $120 billion Iraq war spending bill today that would keep military operations afloat through September.

The bill was a major concession by Democrats who wanted to include a timetable for U.S. troop withdrawals, but relented because they didn't have enough votes to override another presidential veto. The House and Senate both planned to approve the measure by the end of the day.

For his part, Bush agreed to accept some $17 billion in added spending so long as there were not restrictions on the military campaign.

"I hate this agreement," said Rep. David Obey, D-Wis., chairman of the Appropriations Committee.

Obey said the deal was the best that Democrats could manage because "the White House is in a cloud somewhere in terms of understanding the realities in Iraq."

The bill includes the nearly $100 billion that President Bush requested for military and diplomatic operations in Iraq and Afghanistan as well as billions in domestic spending, including $6.4 billion in hurricane relief and $3 billion in agricultural assistance.

Republicans were unhappy about the added domestic spending, but said they were relieved the final measure did not attempt to set a timetable on the war.

"We cannot and will not abandon the Iraqis to be butchered by these terrorists in their midst," said Rep. David Dreier, R-Calif. "And we cannot and will not abandon our mission just as real progress is starting to be made."

The hefty spending bill has become a lightning rod for political attacks on Bush and his handling of the deeply unpopular war, which has killed more than 3,400 U.S. troops and cost more than $300 billion. But it also has exposed a sharp divide among Democrats on how far Congress should go to end the war.

Democratic presidential contenders on Capitol Hill are vying for the anti-war vote, but at the same time do not want to appear as though they are turning their backs on the military.

"I believe as long as we have troops in the front line, we're going to have to protect them," said Sen. Joseph Biden, D-Del. "We're going to have to fund them."

Biden was alone among the potential Democratic candidates in immediately pledging his support for the bill.

Two front-runners, Sens. Hillary Rodham Clinton of New York and Barack Obama of Illinois, declined to say how they intended to vote on the measure.

Both have voted against binding timetables for troop withdrawals in the past, before public sentiment against the war hardened or they became presidential contenders. Last week, the two voted to advance legislation that would have cut off money for U.S. combat operations by March 31, 2008, cutoff.

Challengers Sen. Christopher Dodd of Connecticut and Rep. Dennis Kucinich of Ohio said they would oppose the measure because in their view it issued a blank check to President Bush on the Iraq war.

"Half-measures and equivocations are not going to change our course in Iraq," Dodd said in a statement. "If we are serious about ending the war, Congress must stand up to this president's failed policy now — with clarity and conviction."

While the measure does not include a timetable on the war, it does threaten to withhold U.S. aid dollars for Iraq if Baghdad fails to make progress on political and security reforms. The president, however, could waive that restriction.

Biden said that while he would vote for the measure, he disagreed with the approach because it could hamper the Iraqi government's ability to take on more responsibility.

Democratic leaders planned multiple votes in the House today to ensure the measure would ultimately pass because of disagreements among members on elements of the bill. One vote was to be on war funding, while another would be to approve the extra money for domestic and military-related projects.

While liberal Democrats were expected to vote against the war funds measure, GOP members were expected to make up for the losses. On the added spending, Democrats likely were to be unified in their support for the measure, overcoming GOP objections.

Democratic objections to the immigration reform proposal have focused on its restrictions on the right of legal immigrants to be joined by their families and its preference for high-tech workers. Under the proposal, undocumented immigrants would be able to obtain a probationary card allowing them to live and work legally in the United States, but which would not place them on the road to permanent residency or citizenship. The proposal also seeks to create a temporary guest worker program that would be implemented once the borders are declared "secure." Up to 1.5 million migrant farm-workers can also obtain legal status through an "AgJobs" measure, supported by Sen. Diana Feinstein (D-CA) and Sen. Larry Craig (R-ID). AgJobs creates a five-year pilot program that would grant legal status to those who have worked in US farms for at least 150 days in the last two years.