NEW YORK (AP) _ Republican vice presidential candidate Sarah Palin enters her debate Thursday night with Democratic rival Joe Biden as many voters harbor serious doubts about her readiness for the nation's highest office.

An AP-Gfk poll released Wednesday found that just 25 percent of likely voters believe Palin has the right experience to be president. That's down from 41 percent just after the GOP convention, when the Alaska governor made her well-received debut on the national stage.

Thursday night's debate in St. Louis gives Palin a chance to overcome the doubts in a 90-minute showcase, her first lengthy give-and-take session since joining the GOP ticket with presidential candidate John McCain.

McCain on Thursday dismissed suggestions that he was upset with campaign staff for holding back Palin and not letting her be herself on the campaign trail.

"We let Sarah be Sarah. She's smart, she's tough, she's been in debates before," McCain told "Fox & Friends" on Fox News Channel. "The American people ... the more they see of her, the more they love her, and I'm confident of that at the end."

Palin has granted just a handful of interviews and has appeared at times to be uninformed about national issues. For example, in a CBS News interview aired Wednesday she appeared unable to cite a Supreme Court decision with which she disagreed while saying many decisions had divided Americans. McCain and other Republicans criticized such questions as "gotcha journalism."

"People will have a chance to see her from beginning to end without being edited," former Sen. Fred Thompson, R-Tenn., told CBS' "The Early Show" on Thursday.

"We've all had bad days," Thompson said, "and she's had some bad moments in some of these interviews, just like the rest of us have had."

Sen. Claire McCaskill, D-Mo., a prominent supporter of Democratic presidential candidate Barack Obama's campaign, played up Palin's debate experience from her race for governor as she argued that Palin has sharper skills than she's being given credit for. The Democratic National Committee has e-mailed news stories to reporters describing Palin's able performances in gubernatorial debates in 2006, part of the party's effort to dispel the notion that Palin is a sub-par debater.

"I think Joe obviously has a challenge tonight. The expectations are so low for Sarah Palin and it's difficult for him, I think. No matter what he does he's going to be criticized," McCaskill told CBS. "Sarah Palin will be tough tonight. She's a good debater, she's an effective communicator, and she knows how to throw a punch with a velvet glove and a smile on her face."

In a conference call Wednesday with reporters, McCaskill was blunt about Biden's potential for error: "My friend Joe Biden has a tendency to talk forever and sometimes say stuff that's kind of stupid." Asked to clarify her remarks, McCaskill said she meant them "affectionately."

A poor performance by Biden could cement a negative image for the Delaware senator.

Palin has been preparing at McCain's retreat in Sedona, Ariz. Biden has been undertaking his own intensive preparation near his home in Wilmington, Del., though he went to Washington for Wednesday night's vote on the economic rescue package.

The 90-minute televised debate was to take place at Washington University in St. Louis, with PBS anchor Gwen Ifill serving as moderator. Ifill herself has come under criticism from some conservatives because she is writing a book, "The Breakthrough: Politics and Race in the Age of Obama," on how politics in the black community have changed since the civil rights era. She has said she has yet to write the chapter on Obama and questioned why people think it will be favorable toward the Democrat.

"Frankly, I wish they had picked a moderator that isn't writing a book favorable to Barack Obama," McCain told Fox News on Thursday. "But I have to have confidence that Gwen Ifill will treat this as a professional journalist that she is."