WASHINGTON — With the campaign’s first debate in doubt, the arrival here of both presidential contenders brought chaos and confusion to the capital as a $700 billion bailout deal that appeared at hand broke down amid a flurry of bitter recriminations.

Lawmakers emerged from an unusual White House meeting late in the day worried that a tentative agreement on a Wall Street rescue plan had fallen apart, with House Republicans in opposition. Democratic leaders immediately blamed Senator McCain, throwing a wrench into his dramatic bid to suspend his campaign and broker a bipartisan deal on what would be an unprecedented government intervention in the markets.

“John McCain did nothing to help. He only hurt the process,” the Senate majority leader, Harry Reid, told reporters after the White House meeting, which President Bush convened with congressional leaders from both parties as well as Senators McCain and Obama.

At stake politically in the negotiations is a gamble made by Mr. McCain on Wednesday, when he abruptly announced he would suspend his campaign and call for Friday night’s debate at the University of Mississippi to be delayed pending a legislative agreement on the bailout package.

Both candidates voiced optimism last night that the debate, scheduled for 9 p.m., would go forward. “I’m very hopeful that we can,” Mr. McCain said in an interview on ABC’s “World News Tonight.” Mr. Obama has resisted calls to postpone the debate all along, and he repeated in several televised appearances last night that he intended to be in Mississippi.

A McCain spokesman, Tucker Bounds, said the campaign had sent staff members to Mississippi in preparation for the debate, but he reiterated that Mr. McCain would not attend unless there was a “locked-down agreement” in Washington.

The Arizona senator faced pressure yesterday afternoon from an ally, the Republican governor of Mississippi, Haley Barbour, who held a midday press conference on the university campus to promote the debate. “I expect there to be a debate tomorrow night. I look forward to it,” he said.

Mr. Barbour, a former chairman of the Republican National Committee, said he had not spoken to Mr. McCain, and he declined to weigh in on the political maneuvering between Messrs. McCain and Obama over the event.

Mr. McCain came under attack from Democrats on Capitol Hill and, increasingly, from the Obama campaign over his decision to return to Washington and his threat to skip the debate.

“There is absolutely no reason in the world why Senator McCain should not debate tomorrow, except that he doesn’t want to,” the Democratic chairman of the House Financial Services Committee, Rep. Barney Frank of Massachusetts, said.

The Obama campaign had initially refrained from directly criticizing Mr. McCain’s move, apparently reluctant to rupture the spirit of bipartisanship that both candidates had advocated. But late in the day, Mr. Obama’s press secretary, Bill Burton, sent a missive to reporters disputing the notion that Mr. McCain had suspended his campaign at all. “Make no mistake: John McCain did not ‘suspend’ his campaign. He just turned a national crisis into an occasion to promote his campaign,” Mr. Burton wrote. “It’s become just another political stunt, aimed more at shoring up the senator’s political fortunes than the nation’s economy. And it does nothing to help advance this critical legislation to protect the American people during this time of economic crisis.”

A spokesman for the Republican National Committee, Danny Diaz, responded to the criticism from congressional Democrats by saying: “It is sad commentary on the Democrat Party that the individuals asleep at the wheel as the housing industry went into crisis are now attacking the leader who worked to reform the system.” Mr. Diaz added that the lawmakers were “more interested in carrying Barack Obama’s water than solving a historic economic crisis.”

At midday, it appeared House and Senate leaders had reached a deal even before Messrs. McCain and Obama arrived in Washington. The chairman of the Senate Banking Committee, Christopher Dodd, appeared alongside Mr. Frank and two Republican senators to announce a “fundamental agreement on a set of principles.” But by midafternoon, two House Republican leaders, Rep. John Boehner of Ohio and Spencer Bachus of Alabama, signaled they were not on board, and at the White House, they objected to the Treasury proposal and suggested an alternative plan that would cost less to the taxpayers.

Talks apparently broke down, and the ranking Republican on the Senate Banking Committee, Richard Shelby of Alabama, emerged from the White House to say: “We haven’t got an agreement.”

With that, the accusations began, with the Democratic ire trained on Mr. McCain and House Republicans.

In a press conference after the meeting, Mr. Obama declined to blame Mr. McCain directly, but he offered a hint of “I told you so” as he reminded reporters that he did not want to come back to Washington in the first place. “What I found and I think was confirmed today is that when you inject presidential politics into delicate negotiations, it is not necessarily as helpful as it needs to be,” he said.

The McCain spokesman, Mr. Bounds, defended Mr. McCain’s decision to return, saying the negotiations were “far more successful because John McCain took a lead role.”

“Action was needed. John McCain called for increased participation and delivered,” he said.

Whether he can deliver Republican votes in the coming days, however, may determine the final verdict on his move to upend the presidential campaign.