Obama had said he would attend no matter what McCain decided to do.
Aides to the Arizona senator said he planned to return to Washington Friday night if no agreement had yet been reached on a $700 billion bailout deal that both candidates said was urgently needed.
The debate, expected to reach a huge viewing audience, was supposed to focus on foreign policy, a topic generally viewed as a McCain strength. But given the roiling economic situation, moderator Jim Lehrer of PBS indicated he wouldn't be constrained by the boundaries the two campaigns negotiated months ago.
The debate was crucial for both Obama and McCain, their first encounter before a national audience in a race that remains exceedingly close just over five weeks from Election Day.
The candidates came with different objectives and points to prove.
Obama needed to convince some skeptical voters he is ready to be commander in chief and steward of a precarious economy.
The 47-year-old Illinois senator, whose calm demeanor and youthful cool can make him appear aloof at times, also needed to demonstrate passion and empathy.
The 72-year old McCain, a 26-year House and Senate veteran, hoped to cast himself as an experienced hand in times of crisis while also distancing himself from President Bush, the unpopular leader of his own party. McCain also needed to tamp down concerns about his age by not making the kind of slip-ups that have dogged him at times during the campaign.
McCain generally won praise for his debating skills during the GOP primaries, delivering answers that were crisp and to the point while getting in the occasional zinger. He also displayed flashes of his legendary temper.
Predictably, each campaigns was setting a low bar for its own candidate while inflating expectations for his opponent.
Obama spokesman Bill Burton sent an e-mail to reporters quoting news stories indicating McCain was the stronger debater, particularly on foreign policy.
"If he slips up, makes a mistake or fails to deliver a game-changing performance, it will be a serious blow to his campaign," Burton said of McCain.
McCain, for his part, praised Obama's debate skills this week, suggesting his rival's performances against Hillary Rodham Clinton during the primaries had helped him win the Democratic nomination.
Asked Friday what McCain needed to accomplish in the forum, senior adviser Mark Salter said, "To do well against a guy who's a pretty good debater, show presidential leadership and be able to speak directly to the American people about what he believes."
Both candidates have been rehearsing extensively, Obama prepping with advisers at a resort in Clearwater, Fla., and McCain putting in debate work at his home outside Washington.
The two presidential hopefuls are scheduled to debate twice more, at Belmont University in Nashville on Oct. 7 and at Hofstra University in Hempsted, N.Y., on Oct. 15. Vice presidential contenders Sarah Palin and Joe Biden are to square off in a single debate Oct. 2 at Washington University in St. Louis.