ST. LOUIS - Republican vice presidential nominee Sarah Palin and her Democratic rival, Joe

Biden, are each set to fly into St. Louis this afternoon for a pivotal political showdown tonight that could have a significant impact on the nation's presidential contest.

So perhaps it's fitting that their vice presidential debate at Washington

University is taking place in a state that has a history of siding with the presidential victor - and where the latest polls show Missouri voters in a too-close-to-call split between Republican John McCain and Democrat Barack Obama.

Although Obama and McCain have three debates of their own, it's the vice presidential face-off that has taken on the aura of the marquee event.

"This is the first time it seems that a vice presidential debate might have an impact on the election," said Peter Kastor, a history professor at Washington University.

And most of the credit goes to the Alaska governor, a virtual unknown before she was selected five weeks ago as McCain's running mate.

"This debate is so pivotal because Sarah Palin has been made into a major campaign issue," said Matt Carlson, an assistant professor of communication at St. Louis University. "War, the economy, health care, Sarah Palin."

Palin's admirers credit her with energizing the Republican base and briefly boosting McCain into a lead in national polls early last month.

Palin is expected to attract thousands to a post-debate GOP rally tonight at St. Louis University, just as she did when huge crowds showed up at her two recent stops in St. Charles County and suburban Kansas City.

Chaifetz Arena, where the rally will be held, has been festooned with an electronic ribbon board that declares: "Sarah Rocks!"

"She has become a central part of the McCain campaign in a way that no vice presidential candidate ever has before, including Geraldine Ferraro," said David Lanoue, chairman of political science at the University of Alabama and the author of a book on presidential debates.

But now, the GOP ticket trails Obama-Biden nationally and in most swing states, according to multiple polls. Many analysts tie that drop, at least in part, to Palin.

She's been the brunt of a chorus of criticism, some lobbed by Republicans, who assert that Palin's performance in a few high-profile interviews shows she lacks the gravitas and experience to be vice president - especially since a victory by McCain would make him, at 72, the oldest incoming president.

A new poll by the nonpartisan Pew Research Center released Wednesday showed that only 37 percent of the 1,505 adults said that Palin was qualified to be president, a 15 percentage-point plunge since a month ago.

By contrast, 63 percent said Biden was qualified to be president.

"We will watch the debate with no real interest in what she is saying, but with a sharp focus on how she says what she says," Carlson said. "Will she looked experienced? Will she look like a fish out of water? "

But Palin isn't the only one on stage tonight who could face added scrutiny.

Analysts say that there will be heavy pressure on Biden tonight as well, because of the expectations that he will outperform Palin.

The challenge for Biden, they say, is to avoid the verbal gaffes for which he is known (the Republican National Committee has set up a Web site devoted to Biden's misstatements) and to convey his recognized expertise on foreign policy.

At the same time, Biden has to guard against coming across as overbearing, too talkative or patronizing.

"He's got a bit of a fine line to walk," said Mitchell McKinney, who teaches courses in political communication at University of Missouri and who co-authored a book on presidential debates.

Warned Dave Robertson, a political science professor at the University of Missouri-St. Louis: "He has to be careful to be very respectful to Governor Palin. Democrats do not want the story line to be that he picked on the governor unfairly."

In a two-pronged effort to woo women voters and keep the campaign's preferred focus on the economy, an unemployed woman autoworker - Marie Williamson of Potosi - will sit with Biden's wife and other relatives in the debate audience.

Biden is likely to work in Williamson's plight during his debate comments tonight.

Biden won't be attending the local Democrats' post-debate gathering at the Pageant, 6161 Delmar Boulevard. Biden is flying out of town to spend time with his son, Delaware Attorney General Beau Biden, who is leaving Friday for Iraq.

The younger Biden is a captain in the Army National Guard.

Also in the spotlight tonight, and not necessarily by choice, is moderator Gwen Ifill, a PBS news host.

Ifill has become the subject of a scorching e-mail campaign in recent days orchestrated by Republicans and pro-Palin forces. They question Ifill's objectivity because of her forthcoming book on the evolution of African-American politics.

The book, called "The Breakthrough: Politics and Race in the Age of Obama," is scheduled to be published by Doubleday on Jan. 20, Inauguration Day for the next president.

The book is described by the publisher as an account of how "the black political structure formed during the Civil Rights movement is giving way to a generation of men and women who are the direct beneficiaries of the struggles of the 1960s."

Ifill was selected to be tonight's moderator by the debate commission and the two presidential campaigns.

Ifill told The Associated Press that questions about her impartiality were unfounded and that she hadn't written the Obama chapter yet.

"I've got a pretty long track record covering politics and news, so I'm not particularly worried that one-day blog chatter is going to destroy my reputation," she said. "The proof is in the pudding. They can watch the debate (tonight) and make their own decisions."

The debate airs at 8 p.m. today on all major networks.

Jake Wagman of the Post-Dispatch contributed to this report.