Sens. John McCain and Barack Obama clashed on the economy and foreign policy last night in the debate that almost wasn't.

Through its opening segments, the focus was on economics and the difficult Washington talks over a bailout of the nation's credit markets.

Here, Mr. Obama tried to put Mr. McCain on the defensive, portraying him as the heir to the fiscal polices of an unpopular president and critiquing the Republican's proposals for tax cuts for corporations.

Responding to a question on the crisis talks that dominated the week in Washington, Mr. Obama said, "We ... also have to recognize that this is the final verdict on eight years of financial policies promoted by President Bush and supported by Sen. McCain."

Mr. McCain did his best to take command as the encounter shifted to foreign policy. He repeatedly characterized the Democrat as naive and inexperienced. Again and again, he suggested that his opponent didn't understand a litany of foreign policy issues while citing his own experience and travels around the globe.

"We seem to come full circle again. Sen. Obama doesn't seem to understand or doesn't get it," Mr. McCain said after one more disagreement on their views on Iraq and its place in the nation's overall security challenges.

With almost no flashes of humor, the tone of the 90-minute session was earnest and civil throughout although the rivals' exchanges seemed to betray more tension as the debate neared its end.

Here, after a week in which Mr. McCain had struggled to appear in command of a shifting economic agenda in Washington, the Republican grew more assertive in displaying his foreign policy expertise. While appearing confident in his own analyses of the foreign policy issues, Mr. Obama found himself more often responding to the Republican's criticisms than taking the offensive himself.

In response to the question of debate moderator Jim Lehrer of PBS, the candidates gave cautious replies on whether they would vote for an eventual Wall Street bailout package, although both suggested that they hoped and expected to support a deal that is not yet in clear sight.

Both candidates resisted the moderator's attempts to pin them down on what of their own policy agendas' would have to be delayed or canceled because of the anticipated cost of the bailout. Despite Mr. Lehrer's efforts to get the candidates to interact spontaneously, they often reverted to passages from their standard stump speeches.

Faulting the Republican's promise of corporate tax cuts, Mr. Obama said, "My attitude is we've got to grow the economy from the bottom up," while promising to provide his own tax cuts to 95 percent of working families.

Mr. McCain visited similarly familiar rhetoric as he portrayed himself as a scourge of pork barrel spending while contending that his opponent had made such spending requests totaling "nearly a million dollars for every day he has been in the United States Senate.'

Mr. Obama agreed that congressional spending is ripe for reform but said the amount of money involved in the earmarks targeted by Mr. McCain paled in comparison with the hundreds of billions in federal revenue that would be foregone under his tax cuts.

On foreign policy, the two candidates revisited their central disagreement on the war in Iraq. Mr. McCain criticized Mr. Obama several times for failing to support or acknowledge the success of the surge in Iraq.

Mr. Obama argued that Mr. McCain was wrong on the more basic question of the wisdom of entering that war in the first place. He said that the Republican had erred in the difficulty that faced U.S. troops there, and on issues such as the presence of weapons of mass destruction.

Invoking Gen David Petraeus, who has gotten credit for the success of the surge, and the efficacy of his tactics over the past year, Mr. McCain said. "Sen. Obama refuses to acknowledge that we are winning in Iraq."

"Nobody is talking about losing this war," Mr. Obama countered "What we are talking about is the next president has to have a broader strategic vision."

On this and on a series of other foreign policy issues, Mr. McCain was occasionally patronizing toward his younger opponent.

"I have the ability and the knowledge and the background to keep this country safe and secure," he said. "I don't think I need any on the job training. I'm ready to go right now."

The rivals appeared on the Oxford, Miss. stage after a tumultuous week in a consistently unpredictable campaign. On Wednesday, Mr. McCain made the stunning announcement that he was suspending his campaign and planned to cancel his appearance on the University of Mississippi stage unless the administration and congressional negotiators were able to reach an accord on a bailout measure aimed at calming teetering financial markets.

Mr. Obama rejected the suggestion, insisting that the Wall Street crisis made the need for the candidates to exchange ideas even more urgent.

Answering the invitation from President Bush, Mr. Obama joined Mr. McCain and congressional leaders at a White House meeting Thursday that degenerated into acrimonious exchanges that set back the chances for a deal that at least some lawmakers had described a imminent.

Congressional Democrats blamed the breakdown on what they characterized as Mr. McCain's grandstanding. Republicans, particularly House lawmakers, insisted that despite the optimistic statements of Democrats and some Senate Republicans, no achievable deal had been on the table.

Faced with continued Washington stalemate and his rival's determination to appear on the debate stage, Mr. McCain changed his mind yesterday morning and headed south after issuing a statement saying that sufficient progress had been made to allow him to leave the Capitol.

McCain partisans argued that he had shown leadership at a time of crisis. Democrats mocked his on-again, off-again decisions on the debate, contending that the debate was expected to draw one of the largest audiences in election history in a historic campaign that is expected to set records for voter interest and turnout.

One measure of the public interest in the campaign came during the two nominating conventions when acceptances speeches of the two nominees, as well as that of Alaska Gov, Sarah Palin, the Republican vice presidential candidate, attracted unprecedented audiences.

But given the drama offstage, the audience was likely to extend to the world financial markets. Financiers and central bankers were watching for any clue to the fate of the negotiations in Washington, and, more broadly, to the approach of the next president to an economic climate likely to remain precarious whatever the outcome of the Washington talks.

The long-awaited clash opened an intense phase of the campaign that could be dominated by debate anticipation and reaction for the next three weeks of the fast-closing presidential marathon. Barring the unforeseen, Mr. Obama and Mr. McCain are will meet twice more, at Belmont University in Nashville on Oct. 7, and at Hofstra University in Hempstead, N.Y., on Oct. 15.

It came as polls depicted a contest in which neither candidate had been able to establish a clear lead. Mr. Obama had maintained a slender advantage through the spring and summer, though not as large as many analysts predicted in a contest in which the ailing economy, two long wars and the a historic unpopularity of a president of Mr. McCain's party all seemed to be tailor-made to enhance the chances of a Democrat.

Instead the Republican ticket surged ahead after their convention.

That bounce fell quickly, however, and amid the economic crisis, although that clearly could go either way. Next week, their running-mates Ms. Palin and Sen. Joseph R. Biden Jr. are to face one another at Washington University in St. Louis, on Oct. 2.

By agreement of the negotiators on both sides, the debate was to follow nine-minute segments with Mr. Lehrer's question to the candidates interspersed with opportunities for them to question one another.