Some of McCain's attacks are expected to focus on Obama's Chicago political connections.
The Illinois that Obama wants the voters to see and hear about is the Springfield of Abraham Lincoln. But as the McCain campaign tries to reverse a precipitous slide in the polls, they're citing Obama's relationship with a violent 1960's radical, Bill Ayers, a corrupt political patron; Tony Rezko; and a militant minister, Jeremiah Wright, as evidence of Obama's lack of judgment and character.
"We talk about telling the truth. We hope it lays over everything he says, from the truth about his associations to the truth about his record and the truth about his intentions," said Nicolle Wallace, McCain campaign.
"When the going gets tough, John McCain gets going. And I think that's what you're going to see in the next four and a half weeks," said Sen. Joe Lieberman, (I) Conn., McCain advisor.
Obama says he won't fire the first negative shot, but he will fire the last. And that could include McCain's close relationship with Charles Keating, who was implicated in the massive savings and loan scandal in the 1980s.
"The last time that we had a banking crisis before the one we're going through right now was the savings and loan bailout where Charles Keating and John McCain were players. If that comes up, that's something we talk about," said Dan Pfeiffer, Obama campaign.
"They may decide that it's time to hit McCain back much harder, and I'm sure they have a reservoir of zingers," said Don Rose, political analyst.
Sybril Bennett, who grew up in Waukegan and runs the journalism department at Belmont University. says the voters are desperate for problem-solvers, not mud-slingers.
"People don't want to hear the negative campaigning. Yes, it's traditional. Yes, it's typical. Yes, it's expected. But I think this time it can back fire on either candidate," said Prof. Bennett.
"It's the end of a campaign closing date. You get knifed in the back sometimes. It's considered what campaigns are about," said Lynn Sweet, Chicago Sun-Times reporter.
The town hall format at Belmont University will allow voters to ask questions while NBC's Tom Brokaw moderates. The candidates' third and last debate will be Oct. 15 at Hofstra University in Hempsted, N.Y.
If Tuesday night's confrontation echoes the most recent campaign exchanges, it could be far more personal and pointed than the two men's Sept. 26 encounter. McCain's running mate, Sarah Palin, has raised Obama's ties to 1960s-era radical William Ayers and to the Democrat's former pastor, the incendiary Rev. Jeremiah Wright. On Monday, McCain accused Obama of lying about the Republican senator's record, and asked, "Who is the real Sen. Obama?"
Obama's campaign rolled out a video recounting McCain's involvement in the 1980s Keating Five savings and loan scandal, while Obama himself accused McCain of engaging in "smear tactics" to distract from economic issues.
Both nominees have condemned character attacks in the past, and some supporters are urging them to cool the rhetoric.
McCain in June told reporters, "Americans are sick and tired of the personal attacks, the impugning of integrity" in campaigns.
Obama told an Iowa crowd in January: "We can't afford the same old partisan food fight. We can't afford a politics that's all about tearing opponents down instead of lifting the country up."
Some Republicans, while defending McCain's recent tactics, feel he needs to engage voters on the issues, not character, to overtake Obama. Scott Reed, who managed Republican Bob Dole's 1996 presidential campaign, said of the economic crisis: "McCain is suffering because Americans typically punish the party in power."
McCain's best bet, Reed said, is to show voters "who has the best solutions."
Obama adviser David Axelrod told reporters the Democratic nominee wants to focus on economic issues but "we're prepared for a very aggressive debate" if it becomes more personal. "We're running for president of the United States," he said. "It's a rough, tough pursuit."
The debate was being held at a time most Americans have a dismal view of the country's direction.
A Gallup Poll released Tuesday showed just 9 percent say they're satisfied with the way things are going, the lowest ever recorded in the 29 years Gallup has asked the question. Asked to name the country's major problem, 69 percent said the economy. Next closest: 11 percent cited the Iraq war.
The Associated Press contributed to this report.