Sarah Palin must sound authoritative and authentic. Joe Biden must sound informed and inoffensive. Both need to reach through the television to connect with middle America.

With Wall Street gyrating and voter interest skyrocketing, Thursday night’s televised contest stands as an oddity: a vice presidential debate that could actually matter.

The stakes could hardly be greater, particularly for the Republicans. A host of national and statewide polls released Wednesday showed voters moving toward Democrat Barack Obama over Republican John McCain nationally and in key electoral states.

The debate arrives at a knife’s edge moment for Alaska Gov. Palin, who has seen her early popularity among some voters ebb under a cascade of probing news stories and halting television interviews. The session, aired from Washington University in St. Louis, will be the nation’s first extended look at Palin’s ability to think on her feet.

Any debate is a high-wire act, in this case 90 minutes of close-up camera time in which a whiny sigh, eye roll, or burbling misstatement, can upend a political career. For the vice presidential nominee, there is the added pressure that a mistake will reflect poorly on the running mate.

“First and foremost,” said Democratic strategist Chris Lehane, “vice presidential candidates must abide by the political Hippocratic oath: Do no harm.”

Particularly harmful, Lehane and others say, are mistakes that play into underlying voter concerns: If Palin were to stumble, for example, or if Biden were to come off as patronizing.

“People really look at these choices as another window on the decision-making and philosophy at the top of the ticket,” said Diana Carlin, a professor of communication studies at the University of Kansas who has surveyed political debates since 1980.

For Palin, the imperative is to reverse the slide. Her multi-part interview with CBS anchor Katie Couric prompted even some conservatives to question her readiness.

“The picture that we’ve been given thus far is something of a caricature,” said Whit Ayres, a Republican consultant. Ayres said that Palin should accent her rationale for being on the ticket.

“She doesn’t have to be an expert in the politics of Kosovo or Georgia to be viewed as a competent vice presidential choice,” he said. “What they are going to be looking for is her set of values, her sympathies, her orientation to the world, who she connects with.”

For Biden, the advice is much the same. Although he has spent three decades in the Senate, he must take care not to bludgeon viewers with his knowledge.

Biden, too, has had his share of gaffes recently – misstating the Democratic policy on the use of clean coal, among other things. Likely pounded into Biden’s head during debate prep: Ignore Palin, cut the Senate-speak and emphasize blue-collar roots.

Even debate veterans were having a hard time predicting the outcome.

“This is reality TV … and anything can happen,” said Republican consultant Nelson Warfield.