WASHINGTON (AP) — Running short on time, John McCain has the most riding on the second presidential debate, though Barack Obama will be out of his scripted comfort zone in the town hall-style confrontation. It could be ugly if Monday's tussling is any indication.

Tuesday night's debate comes exactly four weeks before Election Day with a lot going on both inside and outside the campaign: Polling shows Obama approaching the 270 Electoral College votes needed for victory, Wall Street is tumbling even further and both candidates are escalating character attacks.

Their target audience in the debate: the roughly 10 percent of the electorate who are undecided and an additional quarter who say they might still change their minds before Nov. 4.

The debate, at Belmont University in Nashville, Tenn., is supposed to be divided equally between the economy and foreign policy, but given the global financial turmoil, economic questions may well dominate. As markets were plunging in Europe and Asia as well as the U.S. on Monday, the candidates were going after each other.

In Florida, GOP vice presidential nominee Sarah Palin raised Obama's ties to 1960s-era radical William Ayers and to the Democrat's former pastor, the incendiary Rev. Jeremiah Wright. In New Mexico, McCain, himself asked, "Who is the real Senator Obama," referred to him critically as a "Chicago politician" and argued that the Democrat says one thing and does another.

Obama, in turn, asserted in North Carolina that McCain was engaging "in the usual political shenanigans and smear tactics" to distract from economic issues, even as his own aides in Chicago assailed the Republican nominee for "an angry tirade" and went after him for his role in the 1980s Keating Five savings and loan scandal.

McCain, a four-term Arizona senator, is trailing in polls and facing dwindling options to thwart Democrat Obama in an enormously troublesome political landscape for Republicans. Obama, the first-term Illinois senator, wants to solidify his lead and avoid any major debate misstep that could set him back in his quest to become the country's first black president.

Each hunkered down with top aides over the weekend to prepare, McCain at his vacation compound near Sedona, Ariz., Obama in the western mountains of newly competitive North Carolina.

In the 90-minute debate, NBC newsman Tom Brokaw will facilitate questions from the audience as tens of millions of viewers tune in from across the country.

"Generally, the stakes in this are higher for McCain," said Phil Musser, a former executive director of the Republican Governors Association. "It's probably one of the last and most important opportunities for him to lay out an economic vision that resonates with middle America in a format that lends itself to doing just that."

But Republicans and Democrats alike say even a strong McCain performance may not be enough.

"McCain can win the debate, but the trajectory of this election would not be fundamentally altered unless Obama also made a pretty dramatic and serious mistake," said Chris Lehane, a Democratic strategist in Vice President Al Gore's 2000 campaign.

McCain is most comfortable during the give-and-take of question-and-answer events that were a hallmark of his 2000 campaign, and his 2008 primary effort. But his consistency largely depends on his mood. When he's on his game, McCain is witty and charming, filled with ready one-liners and stories from his past. When he's off, McCain can come across cranky, surly and prone to gaffes.

Obama typically is much more at ease giving speeches from behind a lectern, though he has taken impromptu questions from audiences and has grown much more adept at the back-and-forth of voter-question sessions throughout the campaign. The debate provides the professorial Obama with an opportunity to show some and seal the deal with voters still struggling to see him as president.

Criticisms of each other are certain.

McCain "might as well take the gloves off," Palin said Monday, signaling that the GOP nominee may well question Obama's character, record and policies as part of a stepped-up effort to make Obama an unacceptable option for voters. It's also likely Obama will go after McCain anew on the Republican's 90 percent support for President Bush in the Senate, and possibly on his character as well.

Neither, however, can afford to swing so hard that he turns off voters, and the audience-participation format makes it a bit more difficult to fully engage.

Ahead of the debate, an Associated Press analysis based on polling, advertising and interviews with strategists on both sides indicated that Obama was on the cusp of the 270 votes needed to triumph in the state-by-state Electoral College vote count.

He has 21 states with 264 votes in his column or leaning his way, including Iowa and New Mexico. President Bush won both four years ago, but even Republicans concede they are likely to fall to the Democrats this year. Also tilting toward Obama: Minnesota, New Hampshire, Pennsylvania and Wisconsin, all states where Democrat John Kerry was victorious in 2004 and where McCain is competing hard.

That leaves McCain with 23 states with 185 votes in his column or leaning toward him, including three longtime Republican-held states that Obama is trying to swipe: Indiana, Missouri, and Montana.

Just six states , with 89 votes, still appear to be toss-ups — Colorado, Florida, Nevada, North Carolina, Ohio and Virginia — and all are states Bush secured four years ago, underscoring McCain's challenge.

McCain also is trying to win a single electoral vote in one of Maine's congressional districts, while Obama is doing the same in Nebraska.