WASHINGTON — Senator McCain’s decision to suspend his campaign and call for the postponement of the first presidential debate tomorrow night is a high-stakes gamble that puts him front and center in the wrangling over a proposed $700 billion government bailout of the financial markets.

The Republican nominee is bidding for a bold demonstration of nonpartisan leadership on the crisis now roiling the American economy, but analysts warned that it could backfire if voters view the move, which comes as Mr. McCain is slipping in the polls, as a mere political ploy.

Mr. McCain surprised the political establishment — and apparently the Obama campaign — with his announcement yesterday afternoon that he was scrapping campaign events and pulling his advertisements to return to Washington and join the congressional negotiations on the Bush administration’s rescue plan.

“It is time for both parties to come together to solve this problem,” Mr. McCain said in a televised statement from Manhattan. “We must meet as Americans, not as Democrats or Republicans, and we must meet until this crisis is resolved.”

He said he doubted the administration’s plan could pass “as it currently stands,” and urged President Bush to call a meeting of top leaders that included both him and Senator Obama. Acceding to Mr. McCain’s request, the president called Mr. Obama last night to ask the Democratic nominee to join him for a meeting in Washington this morning, according to a statement by the Obama campaign. Mr. Obama agreed to do so, his campaign said.

Mr. Obama earlier in the day appeared cool to the idea of returning to Washington, telling reporters he would do so if congressional leaders thought his presence would be helpful.

Appearing in Florida after Mr. McCain spoke in the afternoon, he rejected his rival’s call to delay the first debate and indicated he would not follow Mr. McCain in suspending his campaign activities or advertising. “I believe we should continue to have the debate,” he said, adding that in the midst of a financial crisis “is exactly the time the American people need to hear from the person who in approximately 40 days will be responsible for dealing with this mess.”

He added: “Presidents are going to have to deal with more than one thing at a time.”

Mr. Obama said he had first called Mr. McCain yesterday morning to suggest putting out a joint statement on the Treasury Department proposal as a show of unity. Mr. McCain did not call him back for six hours, Mr. Obama said, and while the Republican floated the idea of setting aside their campaigns and postponing the debate, Mr. Obama said he did not think Mr. McCain had already decided to do so. Soon after their phone call, Mr. McCain appeared on television and made his own announcement.

The two campaigns did issue a joint statement several hours later. “This is a time to rise above politics for the good of the country,” it read in part. “We cannot risk an economic catastrophe. Now is our chance to come together to prove that Washington is once again capable of leading this country.”

CNN reported last night that the McCain campaign had proposed swapping tomorrow’s presidential debate at the University of Mississippi for next Thursday’s vice presidential debate in St. Louis. The two running mates would then reschedule their debate for a later date, but hold it in Mississippi.

“There has been no final decision at this time,” a McCain spokesman, Tucker Bounds, told The New York Sun. He also denied a report that Mr. McCain would skip tomorrow’s debate if a bailout agreement were not reached.

While Mr. McCain has succeeded in grabbing the campaign spotlight, his decision also weds his immediate political fortune to the fate of a bailout package that many Americans view warily if not entirely unfavorably. A Republican strategist unaffiliated with his campaign, Kevin Madden, compared it to Mr. McCain’s all-in support of the Iraq troop surge, which had been deeply unpopular with the public.

“If he’s going to do it, he’s going to own it as an issue in the campaign,” Mr. Madden, who worked for Mr. McCain’s rival, Mitt Romney, in the Republican primary, said. “He’s very much an instinctual player.”

Just as Mr. McCain had to rely on American military success to justify his support of the surge, his poll numbers may now be dependent on the markets. “If the markets respond well, he will have acted on principle with the expectation that the American people will reward him on that,” Mr. Madden said.

Polls conducted in recent days have shown a trend away from Mr. McCain amid the economic turmoil. Mr. Obama opened up a six-point lead in a national Fox News survey released yesterday, a day after a Washington Post/ABC News poll gave him a nine-point advantage. Two other national surveys showed the Democrat with a smaller edge, but polls in the battleground states of Colorado, Virginia, and Pennsylvania have shown Mr. Obama gaining ground in the last week. The results mark a shift from early in the month, when Mr. McCain picked up a sizable bounce from his selection of Governor Palin as his running mate and the Republican National Convention.

“Clearly it’s an effort on the part of Senator McCain to stop the erosion of his poll numbers and show leadership,” the dean of Baruch College’s School of Public Affairs, David Birdsell, said. “It may in fact boomerang and show the reverse.”

He said voters could recoil at the proposal to postpone tomorrow’s debate, and ask of Mr. McCain, “Who is trying to suspend the fundamentals of the process?”

Democrats in Congress hinted that Mr. McCain’s presence on Capitol Hill would not be welcome, and the Senate majority leader, Harry Reid of Nevada, explicitly warned both presidential candidates to stay away from the Capitol.

“It would not be helpful at this time to have them come back during these negotiations and risk injecting presidential politics into this process or distract important talks about the future of our nation’s economy,” Mr. Reid said in a statement. “If that changes, we will call upon them. We need leadership; not a campaign photo op.”

The market is unlikely to react positively to several more days of bickering and debate in Congress. “Anything that prolongs uncertainty is bad for the markets,” an investment manager who has tracked market patterns during congressional sessions, Eric Singer, said.

Mr. Singer runs a mutual fund called the Congressional Effect Fund that invests in the S&P 500 only when lawmakers are on recess. When Congress is in session, he pulls his investments from everywhere except low-risk money market funds and treasuries.

According to his analysis of 10,000 trading days since 1965, the market has risen at an annualized in price rate of 1.6% when Congress is in session and 17.6% when it is not.