This, apparently, is it.

After eighteen months and 40 (!!!) face-offs, the 2008 presidential debate season comes to a close tonight with the third and final showdown between Messrs. McCain and Obama at Hofstra University in Hempstead, N.Y. This is good news for the Democrat. Ahead in the polls by an average of 7.7 percentage points—by far his largest lead since, well, ever—Obama's only job this evening is not messing up; no more debates, no more mistakes (or at least far fewer people paying attention). McCain, on the other hand, has a much harder mission: catching up. Tonight, 70 million Americans will be watching; the next time either candidate will command an audience that large will be on Jan. 20, 2009. So when the pundits—and, frankly, Team Obama—say that this is the Arizonan's "last best chance" to "change the game," they're not, for once, exaggerating.

The big question is how he'll try to do it. Or, phrased more accurately, who will try to do it. Over the past few days, two McCains have emerged on the campaign trail. The first says he relishes his "underdog" status, pledges to "fight for [his country]" and refuses to impugn Obama personally (policies are fair game). The second is still airing negative ads 100 percent of the time—including a spot that hammers on the Ayers-Obama connection—and promising to bring up the unrepentant radical on stage in New York. "I['ll] I whip his you-know-what," McCain vowed Sunday.

It's easy to see why McCain No. 2 would be tempted to show up tonight and, as NBC's Chuck Todd puts it, attempt to “disqualify Barack Obama.” The truth is, McCain can't win unless Obama is "disqualified." The Democrat currently averages 50 percent of the vote, with 12 of the last 17 national polls showing him at or above that mark. Which means that McCain could capture every remaining undecided voter and still lose the election (were it held today). To come out of top, the Republican needs to convince people who are already supporting Obama to jump ship. Hence "disqualification."

The problem is that McCain has spent the last dozen days monomaniacally attempting to disqualify his rival, throwing Ayers, ACORN and "infanticide" against the wall in the hope that something would stick. Unfortunately, McCain is worse off now than he was when he first launched these attacks. The proof is in the pudding. At the start of October, Obama was beating McCain 48.9 percent to 43.6 percent, according to the RealClear Politics national polling average; since then, Obama's average support has increased 1.1 points to 50 percent while McCain's has slipped the same amount to 42.4 percent. Obama currently holds double-digit advantages in eight national surveys.

A look at the two candidates' favorability ratings explains why. On Sept. 24, 53.8 percent of voters saw Obama favorably and 36.8 percent saw him unfavorably—a net positive rating of 17 points. On that day, McCain's favorable, unfavorable and net-positive numbers were identical to Obama's: 53.8, 36.7 and 17.1. Not anymore. While Obama's net-positive rating has grown to 21.5 points over the past three weeks (57.4 to 35.9), McCain's has plummeted to 7.8. At this point, only the barest majority—50.1 percent, a four-point loss since Sept. 24—see McCain positively. Even worse, McCain's negatives have ballooned by nearly six points over the same period. Legendary GOP strategist Lee Atwater once said that "if you could push an office-seeker's negative number above 40 percent, there would be no way that office-seeker could win his race." McCain's current number? 42.3 percent. 

If McCain wants to come out swinging tonight, fine. Aggression will certainly satiate the die-hard Republicans who've started waving "Take Off [the] Kid Gloves or You'll Lose" signs at his rallies. But as the past two weeks have shown, the voters who matter—late-breakers, undecideds, soft Obamans—have only become less inclined to support McCain as he's turned-up the heat. Nate Silver is right to characterize this as a feedback loop of sorts: "the worse McCain's poll numbers become, the more desperate his campaign looks, and the more desperate his campaign looks, the worse his poll numbers become." Negative messaging doesn't work when voters see the messenger in a negative light—especially as Election Day approaches and the economy crumbles around us.

That's why tonight is a "Mission Impossible" situation for McCain. His best bet, I think, is to angle for a memorable moment that reminds viewers of why they once liked him (and not why they shouldn't like Obama). He can't afford to "win on points" or (God forbid) "tie." McCain's "moment" needs to overwhelm tonight's other 90 minutes. It needs to be ripe for endless recycling on cable news and YouTube. And it needs to be the only thing we remember tomorrow. He could refuse to answer the inevitable question about Ayers, admitting that he reluctantly indulged in such attacks but has now realized that the American people want something more from him. He could announce a bipartisan cabinet. He could fire his staff, take down his ads and declare that he'll be providing the press with unlimited access from now on—as conservative columnist William Kristol suggested on Monday.

Honestly, I don't know. Each of these moments would expose McCain to the usual attack from Team Obama—that he's "erratic"—so I'll leave the strategizing to Steve Schmidt. But the fact is, McCain is no longer in any position (poll-wise) to "disqualify" his rival; from here on out, he'll have to hope that events—a national-security situation, a major gaffe, a dead girl, a live boy—do the job for him. In the meantime, McCain's goal should be "requalifying" himself for the presidency among the dwindling number of voters who say they're supporting Obama but aren't completely sold. The rest is up to fate.

That mission, at least, is still possible. We'll see if he chooses to accept it.