As debate nears, VP hopefuls have something to prove

— -- Republican Sarah Palin and Democrat Joe Biden are hours away from facing off for their first — and only debate, and each has plenty to prove to the millions who will be watching.

The candidates, who debate at 9 p.m. ET at Washington University in St. Louis, have to show they could step into the top job if needed. They must excite the party faithful and entice undecided voters. And they must mind the vice presidential credo: "Do no harm" to the nominee, all while sinking their teeth into the opponent.

Palin and Biden each come with their own specific challenges. Since being chosen by GOP nominee John McCain on Aug. 29, the Alaska governor has inspired excitement within the party — especially among conservatives wary of McCain's work with Democrats on immigration and an overhaul of campaign-finance laws. But no one outside of Alaska has seen Palin debate live, and her recent network TV interviews — in which she struggled to answer questions about the government's financial bailout plan and her self-described expertise on Russia — have been widely lampooned.

Palin's credibility has suffered: In a poll out Wednesday from the Pew Research Center, only 37% of Americans said Palin is qualified to be president compared with 63% for Biden. In a Pew poll three weeks ago, 52% said Palin was qualified.

McCain on Thursday dismissed suggestions that he was upset with campaign staff for holding back Palin from extensive questioning by reporters and voters 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."

Tonight's wild card: How will Palin handle the intricacies of policy without a teleprompter?

Biden, whose choice as Democrat Barack Obama's running mate was far less of a surprise than Palin's, is known for his windiness, zingers and gaffes.

He muddied the launch of his second presidential bid in February 2007 when he was quoted in the New York Observer referring to Obama as "clean" and "articulate." During a primary debate last year, Biden brought down the house by saying that GOP contender Rudy Giuliani, a former New York mayor, was using just three words in every sentence: "a noun, a verb and 9/11."

The unknown for tonight: Whether the Delaware senator will say something condescending, controversial or clumsy.

"Her vulnerability is sounding as if she's reciting something written on an index card," says political scientist Kathleen Hall Jamieson of the University of Pennsylvania. "You listen to Sen. Biden and you think, he's never seen an index card."

Mitchell McKinney, a University of Missouri-Columbia professor who studies political debates, predicts "an uptick" in viewership from previous vice presidential debates. The first of three Obama-McCain matchups last week drew 52.4 million viewers. "We could be in a situation where we actually for the first time eclipse a presidential debate," he says.

Biden eased off his hectic campaign schedule to spend most of the week prepping for the debate at home in Wilmington, Del., where Michigan's Democratic governor, Jennifer Granholm, played the role of Palin.