The debate's town hall format — which will include questions on both domestic and foreign policy raised by audience members at Belmont University in Nashville, Tennessee, and voters participating through the Internet — is McCain's favorite style of campaigning. The second of three presidential debates comes just four weeks before Election Day.
Despite voters rising anxiety about the U.S. financial meltdown, the candidates are also likely to go after each other on character issues, which McCain and his running mate Alaska Gov. Sarah Palin have forcefully re-injected into the campaign since the weekend.
As McCain's poll standings fell along with the economy, his campaign began the new character criticisms and used Palin to spearhead the push. Obama's campaign didn't wait long to respond by resurrecting McCain's links to a financial scandal two decades ago.
Obama and McCain faced cameras Monday with harsh words for each other. Obama, taking a break from debate preparations in Asheville, North Carolina, accused McCain's campaign of "smear tactics" to distract voters.
Obama told reporters McCain was not paying enough attention to the economic crisis gripping the country. The first-term Illinois senator said he could not "imagine anything more important to talk about" than Americans' losing their jobs, health care and homes.
In Albuquerque, New Mexico, McCain delivered an unusually scathing broadside. He accused Obama of lying about McCain's efforts to regulate the home loan industry. And he suggested Obama is a mysterious figure who cannot be trusted.
"Who is the real Barack Obama?" McCain said to a cheering crowd. "Ask such questions and all you get in response is another barrage of angry insults."
Brookings Institution political scientist Thomas E. Mann said he had felt for months that McCain "would eventually have to try to undermine Obama as an acceptable choice for president and commander in chief." Key issues, he said, including "an economy in turmoil, an unpopular war and a politically discredited president are working powerfully against McCain and the Republican Party in general."
Obama, meanwhile, has learned the lessons of Michael Dukakis and John Kerry. Those Democrats lost presidential elections after hesitating to counter hard-hitting and factually dubious attacks on their character and judgment.
"We don't throw the first punch, but we'll throw the last," Obama said Monday on Tom Joyner's syndicated radio show.
The vitriol increased dramatically over the weekend when Palin said the Democratic standard-bearer sees America as so imperfect "that he's palling around with terrorists who would target their own country," a reference to 1960s-era radical Bill Ayers.
Ayers helped found the violent Weather Underground group, whose members were blamed for several bombings when Obama was 8. Obama has denounced Ayers' radical views and activities.
Obama and Ayers live near each other in Chicago, and once worked on the same charity board. Ayers hosted a small, meet-the-candidate event for Obama in 1995, at the start of his political career, but multiple news accounts have said they are not close. The Obama campaign called Palin's remarks outrageous and grossly exaggerated.
The Obama campaign responded by e-mailing a 13-minute Web "documentary" about McCain's involvement with convicted thrift owner Charles Keating, calling the episode "a window into McCain's economic past, present and future."
Just months into his Senate career, in the late 1980s, McCain made what he has called "the worst mistake of my life." He participated in two meetings with banking regulators on behalf of Keating, a friend, campaign contributor and savings and loan financier who was later convicted of securities fraud.
The Senate ethics committee investigated five senators' relationships with Keating. It cited McCain for a lesser role than the others, but faulted his "poor judgment."
With early voting already under way in some states, McCain desperately needs to stem Obama's momentum, but his attacks on the Democrat's character risk alienating the dwindling bloc of undecided voters.
A top independent pollster in the battleground state of Pennsylvania said McCain's attacks are unlikely to change many minds.
"The economy is so dominant and the change focus so great, I just don't think voters are going to buy into it," said Terry Madonna, a political scientist at Franklin and Marshall College.
As the mudslinging intensified, the most recent Gallup Poll daily tracking survey showed Obama up by 8 percentage points, 50-42. It was the tenth day in a row that Obama has held a statistically significant lead in the poll "and the longest for either candidate at any point in the campaign," the Gallup organization said.
Palin was likely to press her attacks on Obama at campaign stops Tuesday in Florida and North Carolina, two Republican-leaning states where McCain is trying to fend off a stiff challenge from Obama.
Palin expanded her attacks on Obama's character Monday to include his relationship with Rev. Jeremiah A. Wright. McCain had signaled earlier he did not want Obama's incendiary former pastor to be a part of his campaign.
As she took up the Wright issue, Palin toned down her description of the Obama-Ayers relationship after her weekend remarks were criticized as exaggerated.
In an interview with New York Times columnist William Kristol, Palin said there should be more discussion about Wright, Obama's pastor of 20 years at Trinity United Church of Christ in Chicago. The Democratic candidate denounced Wright and severed ties with the church last spring after videotapes surfaced showing Wright asking God to "damn" America and suggesting the U.S. government could have spread AIDS in the black community.
Wright had appeared to be off limits for the McCain campaign ever since McCain himself condemned the North Carolina Republican Party in April for an ad that called Obama "too extreme" because Wright was his pastor.
He asked the party to take down the ad and said, "I'm making it very clear, as I have a couple of times in the past, that there's no place for that kind of campaigning, and the American people don't want it."
When Kristol pressed Palin about Wright, she replied, "I don't know why that association isn't discussed more, because those were appalling things that that pastor had said about our great country." She continued, "To me, that does say something about character. But, you know, I guess that would be a John McCain call on whether he wants to bring that up."
Palin also has faced scrutiny over her church practices. A video on her hometown Pentacostal church Web site shows her being blessed three years ago by a Kenyan pastor who prayed for her protection from "witchcraft" as she prepared to seek the governor's office.