When President Bush flies to this Allegheny mountain resort Friday to meet congressional Republicans, he will encounter a party far less malleable and willing to follow his lead than it has been for the past four years.

Bush is accustomed to getting his way with Congress and finished his first term without suffering a major defeat. But mid-level and rank-and-file Republicans have begun to assert themselves on issues including intelligence reform, immigration and a major restructuring of Social Security, the centerpiece of his second-term agenda.

Ways and Means Committee Chairman Bill Thomas (R-Calif.), who has offered a variety of Social Security ideas that differ from the president's, assured Bush at a meeting Wednesday in the White House residence that he is still fighting on his side.

"I've just opened up a new front," Thomas added, according to a participant.

Such independence was much rarer when the party's prospects for keeping control of Congress were tied to Bush's political health, and his reelection was everyone's priority. But now that Bush has run his last campaign, he is being bolder in calling for legislative action than many lawmakers who must run every two years are willing to be.

That leaves the success of his second-term agenda very much in doubt.

In hallway conversations, over glasses of wine and even in front of television cameras, Republican lawmakers are expressing trepidation about some of Bush's plans, putting him in the undesirable position of having to sell himself to his own party when he could be focusing on Democrats and independents.

Many House Republicans are hesitant to do anything that might jeopardize their chances in the midterm elections in 2006, while in the Senate at least half a dozen members have begun jockeying for the White House.

It's the 'no interest like self-interest' rule, and it's every man for himself," said an aide to a Senate Republican committee chairman, who spoke on the condition of anonymity to maintain good relations with the White House. "He's discovering the fine line between having a mandate and being a lame duck."

White House Chief of Staff Andrew H. Card Jr. went to the Capitol on Wednesday as the guest speaker at a regular leadership meeting and to talk about the need for Republicans to be reformers and work together. House Majority Leader Tom DeLay (R-Tex.) said he thinks it is important for Bush to confront the issue of immigration and provide leadership on broad legislation.

Participants said that the tone was respectful and that Card reiterated the administration's commitment to Bush's temporary-worker program and immigration enforcement issues.

After lawmakers took a six-hour train ride from Washington to the Greenbrier resort here, White House senior adviser Karl Rove worked the crowd and gave the first of several presentations, devoting most of his introductory remarks to Social Security. Rove, discussing the issue at the request of congressional leaders, said that taking it on is important and will be popular.

Bush will make his pitch personally to congressional Republicans at a luncheon Friday.

Republican National Committee Chairman Ken Mehlman has begun conferring with lawmakers daily in a bid to sell the president's agenda. He said a main mission is to be a good listener for those who have qualms about Bush's plan to partially privatize Social Security, and to back up worried lawmakers with the party's research, regional media, booking and grass-roots operations.

"Off-year elections are won through the party's ability to motivate the base and persuade swing voters, and this is good politically from both perspectives," Mehlman said.

The skeptics remain vocal, however. During a visit to the White House this week by Finance Committee Republicans, Sen. Olympia J. Snowe (Maine) told Bush she would be concerned about doing anything that would undermine the guaranteed benefit of Social Security.

"We'll keep you in the open-minded camp," replied the ever-optimistic Bush, according to two people who attended the meeting. Later, she told reporters she will oppose the diversion of payroll taxes to individual accounts, the crux of the president's plan as his aides have discussed it so far.

Fifty-five of the Senate's 100 members are Republicans. Sixty supporters would be needed to overcome a delaying tactic known as a filibuster, so Snowe's voice is critical to the GOP. She said in an interview that it was "clear that he [Bush] was soliciting input, recognizing that it is a volatile and sensitive subject where there are disparate views."

"I always tell my colleagues that the Founding Fathers had a great idea, and that was checks and balances," she said.

The White House got a taste of the legislative branch's coming assertiveness late last year, when two committee chairmen temporarily held up a restructuring of the intelligence services -- which the president said he wanted -- because of concerns about a Bush-backed compromise.

Thomas, the House's chief tax writer and a fearless power broker, used an appearance at a National Journal forum earlier this month to announce that he plans to consider a much broader and deeper review of Social Security than Bush has envisioned.

"You people," he said, gesturing toward several former White House officials, "propose; the Congress disposes." He said Bush's failure to veto any bill so far "means we have some latitude in putting together a package that saves Social Security that is perhaps broader than the theme that he is primarily focusing" on.

That theme is a mechanism to allow younger workers to divert part of their payroll taxes into a personal stock-and-bond account. Thomas wants to use the occasion to consider eliminating the payroll tax and to add a savings program for long-term care. At least some House leaders have hailed Thomas's broadside because they believe that Bush's idea alone would fail but that Thomas's expanded ideas might make the plan more attractive to businesses and older Americans.

Sen. Rick Santorum (R-Pa.), a Bush backer and chairman of a Social Security subcommittee, contends that the differences between Bush's needs and those of the GOP in Congress are not enough to create a real fissure. "It isn't about the president personally anymore," Santorum said. "But at the same time, we all know that if the president's not popular and we're not being successful as a party, it hurts us all."

Sen. Trent Lott (R-Miss.) said the divergence of interests between a White House and a legislative majority of the same party "is natural and happens almost inevitably in a second term."

While the White House thinks Social Security legislation will be dead if it is not signed this year, Sen. John W. Warner (R-Va.) said such an undertaking will take some time, "and it should -- it really should."