NEW YORK—Sarah Palin is heading into her debate with Joe Biden, easily the most-anticipated vice presidential faceoff ever, weighed down by fresh evidence that voters are developing serious doubts about her readiness for the job.

A new 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.

There’s a potential bright side: tonight’s debate in St. Louis gives her a chance to overcome the doubts in a 90-minute showcase, the first time most Americans outside Alaska will see her in a lengthy give-and-take session.

The downside: A poor performance debating Biden, the Delaware senator, former presidential candidate and longtime foreign policy expert, could cement a negative image for the rest of the campaign.

Palin has been preparing at Republican presidential candidate John McCain’s retreat in Sedona, Ariz.

Biden was doing his own intensive preparation near his home in Wilmington, Del., though he was going to Washington for Wednesday night’s vote on the economic rescue package.

As for Palin’s prospects, “the expectations are set so low for her, she could fake everyone out,” said Scott Reed, who managed the presidential campaign of Republican Bob Dole in 1996.

“Palin needs to clear the bar and reframe the debate around Barack Obama and his tax and spend record,” he said. “She’s got to show a grasp on the issues and she’s got to talk about Obama. Most importantly, she’s a reformer. She’s got to get back to that.”

Democrats, meanwhile, were doing what they could to dispel the notion that Palin is a sub-par debater. The Democratic National Committee e-mailed news stories to reporters describing her able performances in debates in 2006 when she was running for Alaska governor.