In this campaign of surprises, and no small amount of impetuous desperation, maybe Sarah Palin will announce that she can’t show up for Thursday’s debate because she has a solemn duty to stand on her porch in Alaska and look out for Russian intruders. If she does make it to St. Louis, the sideshow will have a record audience for a vice-presidential encounter. They’ll be coming to see whether Biden will fall into the kind of loose talk that put FDR on television in 1929 (despite the fact that there was no television and he wasn’t President). They’ll come to see whether the improbable Palin will stumble and fall. Unlike the Presidential debate, the drama and melodrama of this one is likely to be confined to the event itself.

Palin enters stage right facing a tough test—and not just because the more the country sees her, the less she seems up to the job and the lower her ratings slip. The ticket she briefly infused with energy now has the pallor of impending defeat after the Obama-McCain meeting at Ole Miss. The unspinnable reality is that Americans thought Obama won—by 12 percent in the latest Gallup Poll.

(I’m amazed by some pundits who proclaimed Friday that there was no clear winner. They’re now repeating their verdicts even as the contrary polling data pile up. Do they think punditocracy is the essence of democracy?)

For Palin, more than for any candidate in modern history, the only defense now is an all-out offense. She will relentlessly attack Obama—with a smile, of course, and her distinctive accent—hoping to crowd out the risk that she will expose her own ignorance or blunder into incredibility amid a wholesale breakdown of syntax. Her survival here depends in large part not on Biden, who has to tread carefully—a tall order for him—but on the questioner, whose role is often underestimated. It was Tom Brokaw who in 1988 asked Dan Quayle a fourth time to justify his experience, provoking Quayle’s disastrous equation of himself to JFK. If Palin plunges off the edge, it’s likely she will have been pushed by a perfectly fair question from moderator Gwen Ifill.

Driving her over a cliff is not Joe Biden’s job. He will avoid condescending or moose-baiting Palin. Instead, he has two fundamental objectives: to stand up for Obama and to go after McCain. That’s what vice-presidential candidates are for, although it didn’t appear that way in the last two races. Joe Lieberman fell startlingly short in a debate where his principal effect was to be nice to Dick Cheney and make Cheney look nice in turn; John Edwards was half-hearted, probably because he had half an eye on 2008.

For Biden, the only future is now. His task is not to prove he’s the brightest boy on stage. (He’s the only one, after all). He’s not there to burnish his foreign policy credentials, but to vouch for Obama’s; to strengthen Obama’s blue-collar appeal, not just flourish his own.

This strategic framework, I predict, will also inform Biden’s tone. Instead of telling Palin she “seems unaware” or “doesn’t know the facts,” he’s likely to choose understatement—“the fact is” or “what really happened here”—which over 90 minutes can be more devastating as well as less dangerous. As McCain’s performance in Oxford proved, there’s no profit in coming across as disdainful and there will be nothing but backlash if the disdained one is a woman. Biden wins if he makes a persuasive case for Obama on national security and holds or widens the Democratic advantage on the economy. Answering attacks on Biden’s own record, the decoy Palin may be programmed to launch, is of tertiary importance at best. The debate’s not about him.

Frum has been clinging to national security as McCain’s life raft and the Republicans are sure to keep pushing the point that Obama’s not ready—this from the party that gave us Palin. It’s a threadbare argument, shredded by Obama in the first debate. Its tattered remains can be swept away if Biden maintains a disciplined approach.

Then, as I’ve written before, we’ll be headed for the swamp. Frum’s colleague Bill Kristol suggested that a beleaguered McCain can always “re-introduce” Jeremiah Wright. I suspect the voters, increasingly fearful about the future and increasingly confident about Obama, will react with a dismissive question: “Is that all you got?” After which the Straight Talk Express will sink in its own muddy quicksand.