A month after Gov. Sarah Palin joined Senator John McCain’s ticket to a burst of excitement and anticipation among Republicans, she heads into a critical debate facing challenges from conservatives about her credentials, signs that her popularity is slipping and evidence that Republicans are worried about how much help she will be for Mr. McCain in November.

Ms. Palin, the Republican vice-presidential nominee, flew to Mr. McCain’s ranch in Sedona, Ariz., on Monday for three days of preparation with a team of his aides — a sharp contrast to the less structured preparation that led up to the senator’s first debate.

The amount of time and staff power being devoted to this was evidence of concern among Mr. McCain’s associates that Ms. Palin’s early triumphs — a well-received convention speech, her drawing of big crowds — have been overtaken by a series of setbacks, creating higher stakes for her in the debate Thursday with the Democratic nominee for vice president, Senator Joseph R. Biden Jr. of Delaware.

“I think she has pretty thoroughly — and probably irretrievably — proven that she is not up to the job of being president of the United States,” David Frum, a former speechwriter for President Bush who is now a conservative columnist, said in an interview. “If she doesn’t perform well, then people see it.

“And this is a moment of real high anxiety, a little bit like 9/11, when people look to Washington for comfort and leadership and want to know that people in charge know what they are doing.”

Ms. Palin, of Alaska, continues to draw large crowds, is helping Mr. McCain with fund-raising and drawing volunteers, and is drumming up support among base Republican voters who were once skeptical of his candidacy, party leaders said in interviews. Yet these rough two weeks have led some Republicans to reconsider their initial assessment that she would sharply increase Mr. McCain’s appeal among women and independent voters.

Her halting interview with Katie Couric on CBS News alarmed many Republicans and gave fodder for a devastating parody on Saturday Night Live.

“I think the Katie Couric interview shows that she needs to be briefed more on certain aspects,” said Jim Greer, the Republican chairman in Florida. “She continues to be viewed very positively by the base of the party, but she needs to demonstrate that she’s got the knowledge and ability to be president should the need arise.”

Polling suggests that the number of Americans who think she is not fit to be president has increased since her introduction to the country last month. A number of conservative columnists and thinkers have publicly turned against her, or criticized Mr. McCain for choosing her, including George Will, David Brooks and Kathleen Parker, who wrote a column entitled “She’s Out of Her League” for the National Review Online.

Mr. Frum noted the difficulty that Dan Quayle, who was elected vice president in 1988, had in recovering from an early set of mistakes that led him to be ridiculed as an intellectual lightweight. “The story of Dan Quayle is he did probably 1,000 smart things as vice president, but his image was locked in and it was very difficult to turn around,” he said. “And Dan Quayle never in his life has performed as badly as Sarah Palin in the last month.”

Several Republicans said that all of this could ultimately play to Ms. Palin’s benefit, lowering expectations for her so much that a mediocre performance in the debate could be hailed as a success.

“Thanks to the mainstream media, quite a low expectation has been created for her performance,” said Ron Carey, chairman of Minnesota’s Republican Party. “The style of Sarah Palin is going to amaze people. She is going to be able to amaze people with the substance she is going to deliver.”

And Mr. McCain’s aides disputed the expressions of concern and said that if anything, the barrage of criticism and the performance in the few television interviews she has done gave her a low bar to clear in the debate. “I seriously hope that people continue to underestimate the most popular governor in America and a woman who speaks to the heart of America’s economic angst,” said Nicolle Wallace, a senior adviser to Mr. McCain.

And Mr. McCain, appearing with Ms. Palin in another interview with Ms. Couric on Monday night, offered a hearty endorsement. “I’ve seen underestimation before,” he said. “I’m very proud of the excitement that Governor Palin has ignited with our party and around this country. It is a level of excitement and enthusiasm, frankly, that I haven’t seen before.”

Ms. Palin is getting ready for the debate at a time of enormous uncertainty about a highly complicated issue, the unfolding crisis on Wall Street, which makes preparing for the face off especially hard.