The newly crowned front-runner in the Republican presidential race, Senator McCain of Arizona, and his nearest rival in the delegate count, Mitt Romney, are clashing bitterly in the lead-up to next week’s pivotal Super Tuesday primary.

Mr. Romney, clearly smarting from his loss in Florida Tuesday, last night accused his opponent of “dirty tricks,” while Mr. McCain replied that the charge lacked credibility coming from the former Massachusetts governor, who has spent millions on television ads attacking other hopefuls.

During the last debate before the February 5 primary, a 90-minute exchange at the Ronald Reagan Library in Simi Valley, Calif., Mr. Romney faulted Mr. McCain for claiming in recent days that the former governor gave an interview in April 2007 in which he endorsed a secret timetable for withdrawing American forces from Iraq.

“I’ve never suggested a specific date to withdraw,” Mr. Romney said. Mr. McCain had months to raise the issue and did not do so until “a few days before the Florida primary, when there was very little time to correct the record,” the former governor complained. “It sort of falls in the kind of dirty tricks that I think Ronald Reagan would have found to be reprehensible,” Mr. Romney said, as Nancy Reagan looked on from the front row of the audience.

Mr. McCain insisted that his charge was fair, in part because any positive reference to “timetables” last spring was in the context of a Democratic-led effort to set a date for withdrawal. “Timetables was the buzzword for those who wanted to get out,” the Arizona senator said. He also broadened his attack on Mr. Romney by noting that, in December 2006, Mr. Romney said he had no position on the surge.

Mr. McCain said his opponent was not entitled to the benefit of the doubt because he showed no such grace in the attack ads he bought. “A lot of it is your own money. You’re free to do with it what you want to. You can spend it all. But the fact is that your negative ads, my friend, have set the tone, unfortunately, in this campaign,” the senator said.

While Mr. Romney was pressed on his national security qualifications, Mr. McCain was asked why he was qualified to shepherd the economy. “I know how to lead. I led the largest squadron in the United States Navy and I did it out of patriotism, not for profit,” he said, in an apparent dig at the former governor’s record as a business consultant. “And I can hire lots of managers,” the senator added.

Mr. Romney suggested that Mr. McCain was insulting the business owners who fuel America’s economy. “We shouldn’t demean the people who are starting up small businesses,” the former governor said. He said not serving in the military was one of his “two great regrets in life,” but that legislators often have an unusual idea of what it means to lead. “They’re committee chairs and they call that leadership,” the ex-governor said.

During exchanges about the economy, Mr. McCain unexpectedly gave the most full-throated endorsement of President Bush’s economic record, while Mr. Romney and two other candidates, Michael Huckabee and Rep. Ron Paul of Texas, were more critical.

Mr. Romney warned that Mr. McCain’s legislation to impose a so-called cap and trade system on greenhouse gases showed his lack of economic acumen because it would place huge burdens on the American economy. “If you do it unilaterally, without involving all the world, you’d impose on the American people a huge new effective tax: 20% on utilities, 50 cents a gallon for gasoline,” the former governor said. He said polluting industries would just move to China or India, where they would emit just as much pollution. “That’s why you’ve got to have a president that understands the real economy,” Mr. Romney said.

All four candidates said they would permit California to impose stricter controls on greenhouse gases from cars — regulations that have been blocked by the Bush administration.

Mr. Huckabee again touted infrastructure spending as better medicine for the economy than the stimulus package Congress is considering. Mr. Romney replied that the highway expansions would take months or years and be too slow to relieve the current problems. However, by choosing Boston’s “Big Dig” to illustrate that problem, the former Massachusetts governor may have stumbled. He noted that the new tunnels leak “badly” and can be considered “the biggest car wash in America and the most expensive, too.”

“It was very badly managed,” Mr. Romney noted, a fact that may not have been his fault but could undermine his reputation as a sterling manager.

Governor Schwarzenegger, who was seated next to Mrs. Reagan at the debate, was expected to endorse Mr. McCain today at an event in Los Angeles. The Arizona senator also picked up a key endorsement yesterday from Mayor Giuliani, who formally ended his bid after coming in third in Florida.

“John McCain is the most qualified candidate to be the next commander in chief of the United States. He is an American hero and America could use heroes in the White House,” Mr. Giuliani said. “He’s a man of honor and integrity, and you can underline both.

The former mayor said he planned to campaign for Mr. McCain, especially in states such as New York and California. However, Mr. Giuliani’s most immediate and significant contribution may be to pass his donor list to Mr. McCain, whose campaign has been strapped for money. Many of the former mayor’s financial backers had “maxed out” to his bid, but could still give the maximum $2,300 donation to the Arizona senator.