With polls showing the race tight, and the debates expected to draw millions of TV viewers, they could tip the balance on Nov. 4.
The pressure probably is greater on Obama, who built his meteoric career largely on charisma and a gift for words.
"The debates are particularly important for Obama," said political scientist Bruce Cain, director of the Washington program for the University of California at Berkeley. Obama's candidacy relies heavily on his promise to break from President Bush's domestic and foreign policies, he said. Such topics are conducive to a debate's thrust-and-parry format, he said, and Obama must capitalize.
"He needs to really identify how he's different on the economy from both the current administration and McCain," Cain said. Obama has emphasized that message for months. But a debate's intimate setting may give it more resonance than the big-stadium speeches many voters associate with Obama.
McCain's candidacy, Cain said, rests more on his image as a corruption fighter and war hero who survived a Vietnamese prison camp. Those qualities are certain to come out during the debates, but they could lack the specificity or immediacy that voters want, he said.
McCain needs a solid debate performance to help sustain the energy boost he got from his choice of Sarah Palin as a running mate; Democrats, for their part, see the series of three debates over 20 days as an opportunity to redirect momentum to Obama. Sen. John Thune, a South Dakota Republican who backs McCain, agrees that Obama carries a heavier burden. Obama has not been on the national stage as long as his opponent, Thune said, and voters have a flimsier grasp of who he is.
"Obama really has to score a punch," Thune said. "He hasn't closed the deal with a lot of American people."
Thune thinks McCain may benefit from low expectations, because Obama is seen as a great orator, a skill that some voters might associate with televised presidential forums even if the comparison is questionable.
Obama's less-than-overwhelming performances against Hillary Rodham Clinton and other Democrats during the primary season showed that the format "was not his strength," Thune said. On the other hand, he said, McCain "is wily, he's effective, he carries questions well," and may exceed many viewers' expectations.
But McCain must do more than repeat some well-rehearsed sound bites and calls for commissions to recommend ways to shore up the nation's housing and investment sectors, said Rutgers University political scientist Ross K. Baker.
When McCain sits or stands next to Obama, 47, the age difference will be obvious, Baker said.
McCain needs to show skeptical voters "how fast his neurons are firing," he said.
"He must show himself to be mentally agile," Baker said, and not too reliant on familiar phrases and punch lines.
Obama, meanwhile, must avoid appearing too aloof and "professorial," Baker said, poking at his own line of work. He said voters want "to be reassured that he understands the kind of day-to-day problems that ordinary people encounter."
McCain's toughest challenge may to prevent Obama and the moderators from pinning him too tightly to the policies and legacy of Bush, whose approval ratings have plummeted in the past two years as the Iraq war drags on and the U.S. economy falters.
McCain is trying to thread a political needle, as the debates are likely to illustrate. He stops short of expressly rebuking Bush, which would anger conservatives and subject him to accusations of flip-flopping. But he argues that he and Palin are feisty mavericks better suited to shaking up Washington than are Democrats Obama and Joe Biden.
Look for McCain to emphasize basic Republican principles, such as low taxes, without mentioning Bush's name. He needs to frame the contest as being between himself and Obama, looking to the future and not to the past of Bush's record, said Sen. Lindsey Graham, R-S.C., one of McCain's closest advisers.
McCain, he said, must show "that the policy differences are real and that our solutions to our economic problems and crises will make us more competitive in the global economy."
To do that, McCain will have to offer more detailed plans than he has thus far, said Cain, the Washington-based California professor. "He has to get much more specific" and go beyond the shorthand he often uses in economic discussions, Cain said. "Just blaming corruption isn't going to do it."
The Oct. 2 vice presidential debate between Palin, Alaska's governor for two years, and Biden, Delaware's senator for 36 years, may draw nearly as much attention as the three McCain-Obama forums. No one expects Palin to match Biden's knowledge of federal matters, particularly his specialty of foreign policy. But Biden, whose verbosity sometimes gets him in trouble, will have to avoid any hint of patronizing Palin.
"Biden is going to have the toughest time," Baker said. He sometimes "lays down a smoke screen of rhetoric" sprinkled with senatorial jargon, Baker said, which could give Palin a chance to appeal to voters with a straightforward, anti-Washington message.
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On the Net:
McCain-Palin: http://www.johnmccain.com
Obama-Biden: http://www.barackobama.com
