The decision could heighten the importance of the Values Voter Summit, to be headlined by James Dobson of Focus on the Family. Dobson has expressed doubts about several of the leading Republican contenders. But they nonetheless are planning to show up at the gathering because of the belief that evangelicals could hold the key to the GOP nomination.

Brownback's positions dovetail with those of the evangelical leaders, but he has failed to make headway because of the perception that he had little voter support. Brownback's withdrawal is expected to help former governor Mike Huckabee of Arkansas, who has a similar political philosophy and who has been "fishing from the same pool" for voters, according to Chuck Hurley, one of Brownback's closest friends and a key Iowa backer. Brownback is pulling out because "he doesn't have the name ID or connections . . . or money" that other candidates have, Hurley said in a telephone interview yesterday.

Meanwhile, Mitt Romney, a former governor of Massachusetts, could benefit indirectly because Brownback will no longer be on the debate stage as a vocal critic of Romney's switch to an antiabortion position.

Brownback had used some of his meager campaign resources to pay for automated phone calls to Iowa voters that portrayed Romney as being "proabortion" as recently as 2005. Romney called Brownback's attack "desperate" during an Aug. 5 debate, but Brownback defended the calls.

Romney, in his speech to the evangelical leaders tonight, does not plan to use the occasion to give a long-anticipated address about his Mormon faith, said spokesman Eric Fehrnstrom. "This is not a religion speech," he said.

Instead, the speech is expected to focus on Romney's proposals for strengthening families. The Romney campaign last night provided excerpts of the speech, quoting Romney as saying he is "pleased that so many people of many faiths have come to endorse my candidacy and my message."

The religious leaders hope to influence the campaign, but support among evangelicals is split widely among the top candidates and it is not clear that they will rally behind one candidate.

For example, while Rudy Giuliani has been criticized by evangelical leaders for his support of abortion rights, he leads Republicans among people who say they attend church weekly, with 27 percent support, followed by Fred Thompson at 24 percent and John McCain at 17 percent, according to a recent Gallup Poll. Romney came in fourth place among regular church-goers at 9 percent, followed by Huckabee at 7 percent.

This week, Romney won the endorsement of Bob Jones III, the chancellor of a fundamentalist Christian university in South Carolina that bears his family's name, but it was far from enthusiastic. "I'd rather endorse someone whose religion is wrong than somebody who doesn't have any religion at all," Jones said in announcing his support.

Romney yesterday said he wasn't bothered by the comment because he and Jones agree on many issues.

"We want marriage before babies," Romney said, according to the Associated Press. "We have the same things we want to fight for on issue after issue, so I'm happy to have his support."

Brownback, who hoped to be the evangelicals' candidate, has been stuck at 1 percent to 2 percent in national polls. During the third quarter of 2007, he raised about $925,000, less than his six GOP rivals, and had less than $95,000 in the bank by Sept. 30.

In a meeting with the Globe's editorial board earlier this week, Brownback was sober about his campaign standing, admitting "we've languished" since he finished third, behind Huckabee and Romney, in the Ames, Iowa, straw poll in August.

Brownback said his support for a guest-worker program for immigrants has hurt him among Republicans.

He also talked about trying to gain traction with his antiabortion views and by pushing a congressional apology for slavery. "If we can't, we won't be able to move forward," he said.

Brownback, who left his evangelical church four years ago and became a Catholic, had hoped to take advantage of concerns among some religious conservatives about some of his rivals.

Thus, Brownback found himself in a political Catch-22: Many evangelical leaders won't back him out of concern he has no chance, and he has little chance without strong support from evangelical leaders.

The Kansas City Star and Associated Press reported that Brownback will formally announce his withdrawal this afternoon in Topeka, and may indicate whether he plans to run for governor in 2010.