In a 90-minute televised debate, the two candidates repeatedly clashed over their economic and foreign policy approaches in a series of sharp exchanges that highlighted broad gulfs in their policy views.

But both men said they were optimistic Congress would come up with a $700 billion rescue plan for US financial services but agreed the huge price tag would limit their agendas as president.

McCain said he would freeze federal spending as president on most programs other than defense and veterans' care, and accused Obama of being a big-spending liberal who could not bring together Republican and Democrats.

"Senator Obama has the most liberal voting record in the Senate," McCain said. "It's hard to reach across the aisle from that far to the left."

Obama said McCain would cut taxes for the wealthy and slash corporate tax rates. He repeatedly tried to tie McCain to the policies of President George W. Bush.

"This is a final verdict on eight years of failed economic policy promoted by George Bush and supported by Senator McCain," Obama said of the economic crisis on Wall Street.

Obama, an Illinois senator, said he would not be able to do everything he wanted in his administration because of the bailout, but said McCain's proposal to freeze spending was "using a hatchet when you need a scalpel."

McCain ended days of suspense earlier on Friday when he flew to Mississippi for the debate, backing away from his promise to skip the showdown if negotiations were not completed on a rescue of the US financial industry.

The Arizona senator's campaign said enough progress had been made that he could participate in the first presidential debate, which could help decide a tight race in the Nov. 4 presidential election.

"We have finally seen Republicans and Democrats sitting down and negotiating together and coming up with a package," McCain said.

The debate was scheduled to focus on foreign policy and national security, but the turmoil on Wall Street has dominated the campaign trail for nearly two weeks and was the first topic raised.

White House talks among McCain, Obama and congressional leaders ended in disarray on Thursday with no agreement on a stalled $700 billion bailout plan proposed by the Bush administration.

'OUT OF CONTROL'

McCain, who has fought local pet spending projects in Congress, said spending restraint would be a vital part of any economic recovery. "The reason, one of the major reasons why we're in the difficulties we are in today is because spending got out of control," he said.

Obama said McCain had backed the spending plans of Bush and Republicans. "John, it's been your president who you said you agreed with 90 percent of the time who presided over this increase in spending," he said.

The debate was expected to be watched by far more than the 40 million Americans who saw the convention acceptance speeches of McCain and Obama, and could be a crucial factor for undecided voters in the Nov. 4 election.

Public opinion polls have shown Obama making gains over the past week on the question of who could best lead the country on economic issues, and most polls have Obama holding a slight and growing lead over McCain.

When the debate turned to foreign policy, Obama criticized McCain's judgment in supporting the US military invasion of Iraq. McCain has been one of the most staunch advocates of the war.

"The first question is whether we should have gone into this war in the first place," said Obama, an Illinois senator who opposed the war from the beginning.

McCain said the big question facing the next president will be "how we leave and when we leave." He said Obama had shown poor judgment by opposing the surge policy that sent more troops to Iraq and has been credited with reducing violence there.

McCain also attacked Obama for his willingness to talk with leaders of hostile nations like Iran without preconditions, but Obama pointed out that Henry Kissinger, the former secretary of state who is one of McCain's foreign policy advisers, supported the same approach.

The two candidates also clashed on Pakistan, with Obama saying the United States should attack militants in Pakistan if Islamabad was unwilling to do so. McCain said he would not back such a policy.