"There's a long way to go," said Representative Tom Davis, Republican of Virginia. "I'm certainly not there yet by a long shot."

Another Republican, Senator Lindsey Graham of South Carolina, warned that Democrats would derail the plan if they successfully made the argument that it would explode the deficit.

"If we are arguing about the effect personal accounts have on the deficit, then we lose," Mr. Graham said. "The biggest Achilles' heel we face as reformers is transition costs."

And Senator Tom Coburn, a newly elected Republican from Oklahoma, said: "Nobody said this was supposed to be easy. Nobody said you were supposed to get automatically re-elected if you do the right thing."

From the moment Mr. Bush turned to the subject of Social Security in his speech, there was no doubt of the intensely partisan battle his proposal had spawned. Democrats hollered "no, no!" as a Mr. Bush asserted that the Social Security system "would be exhausted and bankrupt" in 40 years, making it appear for a moment that Mr. Bush was standing in the well at the British House of Commons. 

Democrats said they would set out to undercut Mr. Bush's proposal with a broad, concerted attack coinciding with Mr. Bush's trip selling the program across the country. They began with speeches and statements on Wednesday, and on Thursday Democratic senators plan to hold a news conference at the Washington memorial to Franklin Delano Roosevelt, the originator of the program.

"The president's plan is so dangerous," said Senator Harry Reid, the Democratic leader. "There's a lot we can do to improve Americans' retirement security," he said, "but it's wrong to replace the guaranteed benefit that Americans have earned with a guaranteed benefit cut of 40 percent or more. Make no mistake, that's exactly what President Bush is proposing."

Representative Sander M. Levin, Democrat of Michigan, warned that Mr. Bush's proposal would result in "massive benefit cuts and massive borrowing."
"Far from resolving the long-term challenges facing Social Security in 40 to 50 years, the president's privatization would make it worse, and bring about the dismantling of Social Security through benefit cuts and massive borrowing," Mr. Levin said.

Many Republicans moved to rally around Mr. Bush and the first State of the Union speech of his second term, though the enthusiasm was clearly much more evident when he talked about Iraq than when he discussed Social Security, the issue with which he is seeking to define his second term.
 
While some Republicans were uncomfortable with the deficit argument pushed by Democrats in trying to discredit the Social Security idea, a number argued that the short-term cost of Mr. Bush's program was trumped by the need to ensure its long-term fiscal solvency.

"Social Security has to be addressed in the context of generations, not in the context of five years," said Senator Judd Gregg, Republican of New Hampshire.

But there were Republicans who grew visibly uncomfortable as Mr. Bush talked about overhauling Social Security. After the speech, a number of Republicans who said they had been on the fence on the issue said their minds had not been changed.

When Mr. Bush declared that the best way to change the system for young workers was to create personal retirement accounts, Senator Olympia J. Snowe, a moderate Republican from Maine who has voiced skepticism about Mr. Bush's plan, perched on her seat, smiling but not clapping, as the rest of her Republican colleagues rose around her.

"We can't just rush pell-mell into making dramatic changes," Ms. Snowe said earlier in the day at a Senate Finance Committee session where Democrats pummeled the program. "We have to exercise due diligence with respect to this system."The signs of Republican queasiness came as Democrats said they sensed vulnerability as Mr. Bush pushed ahead with his ambitious plan, and would keep trying to put Mr. Bush and Republican majorities in both houses on the defensive.

"It is all Social Security, 24/7," said Representative Rahm Emanuel of Illinois, the new chairman of the Democratic Congressional Campaign Committee.
Anne E. Kornblut and David D. Kirkpatrick contributed reporting for this article