Tonight's format, with direct questions from voters, could benefit McCain, who prefers that setting. But this may be the only built-in advantage for the Republican, who trails in polls amid a darkening economic climate.
Obama says McCain is desperately trying to distract voters from the economy by employing smear tactics. Many Democrats are worried that they might work.
In a radio interview, Obama tried to reassure supporters.
"One of the things we've done during this campaign, we don't throw the first punch but we'll throw the last," he said on the Tom Joyner Morning Show, which claims a largely African-American audience of 8 million.
McCain is attempting to tie Obama to controversial Chicago figures and accusing the Democrat of not being open with voters about his past.
Obama said that "if John McCain wants to have a character debate, then I'm happy to have that debate, because Mr. McCain's record, despite him calling himself a maverick, actually shows that he is continually somebody who relies on lobbyists for big oil and big corporations, and that he makes decisions oftentimes on what these lobbyists tell him to do."
McCain and running mate Alaska Gov. Sarah Palin were resorting to negative campaigning "because they don't want to talk about the economy and the failed policies of the last eight years," Obama said.
A few hours later, Obama opened a new character attack of his own, linking McCain's involvement in the Keating Five scandal of the 1980s to the latest financial crisis. Also yesterday, Obama released a new attack ad against McCain, calling him "erratic" and as "out of touch" as President Bush.
McCain, in turn, took a personal swipe at Obama, using the word "angry" to describe the first African-American on a major party ticket.
At a campaign rally in the battleground state of New Mexico, McCain echoed his own latest negative TV ad, which attempts to exploit voter doubts about Obama. The commercial begins with a narrator asking "Who is Barack Obama?" and calls the Democrat "dishonorable," "dangerous" and "too risky for America."
"Who is the real Barack Obama?" McCain said to supporters in Albuquerque. "My friends, you ask such questions and all you get in response is another angry barrage of insults."
Palin went after the Democrat at a campaign event in Florida, another Republican state in 2004 that is in the tossup category this year.
She said Obama is "someone who sees America as imperfect enough to work with a former domestic terrorist who targeted his own country." It was the latest volley in an attack that Palin began over the weekend, appearing to question Obama's patriotism by claiming that the Illinois senator pals around with terrorists.
Her references were to Bill Ayers, a University of Illinois professor who was an early Obama supporter in Chicago and has been an occasional social acquaintance and professional associate. Ayers was a founder of the Weather Underground, a violent antiwar group whose bombings caused property damage and killed a police officer in the early 1970s.
Obama has denounced Ayers' radical activities and pointed out that they took place when Obama was 8. Ayers contributed $200 to Obama's state legislative re-election campaign in 2001.
Speaking to reporters in Asheville, N.C., where he was rehearsing for tonight's debate, Obama said McCain and Palin want to "brush aside" economic issues and, instead, "engage in the usual political shenanigans and smear tactics that have come to characterize too many political campaigns, [which] is not what the American people are looking for." He ignored questions from reporters, however, about his campaign's new character counterattack against McCain.
It came in the form of a 13-minute online video that assails McCain for his association with convicted savings-and-loan figure Charles Keating Jr., a major McCain donor who went to prison on fraud charges.
In 1991, the Senate Ethics Committee rebuked McCain for "poor judgment" in meeting with federal regulators on the businessman's behalf, along with four other senators, in what came to be known as the Keating Five scandal.
McCain had received $112,000 in campaign contributions from Keating and his associates. After Keating's activities came under government scrutiny, McCain belatedly repaid more than $13,000 for private trips taken with the Arizona businessman.
McCain later called his meeting with regulators "the wrong thing to do." In an effort to redeem his image, he opened a new phase in his career as a political reformer and became the co-author of a 2002 law that overhauled the federal campaign finance system.
In releasing the video, the Obama campaign called the Keating scandal "eerily similar to today's credit crisis, where a lack of regulation and cozy relationships between the financial industry and Congress has allowed banks to make risky loans and profit by bending the rules." Obama campaign manager David Plouffe, in an e-mail to supporters, called the scandal part of a "pattern of poor judgment by John McCain."
At 9 tonight, the candidates will face questions from an audience of undecided voters at Belmont University in Nashville, Tenn. The moderator, Tom Brokaw of NBC, has the discretion to include questions submitted online as well.
McCain, who prefers town-hall style events to more traditional rallies, managed to rescue his candidacy during the primaries, at least in part, through his performances at anything-goes sessions with voters. During the summer, Obama rejected a McCain challenge to meet him in a series of 10 town-hall debates around the country.