"I have never, ever supported a specific timetable" for withdrawing troops, Romney said. McCain's accusation, ahead of Tuesday's primary, he said, "sort of falls into the dirty tricks that I think Ronald Reagan would have found reprehensible."
The debate was held in the Ronald Reagan Presidential Library in Simi Valley, Calif., six days before more than 20 states hold primaries or caucuses that could determine who succeeds President Bush as the party's standard-bearer.
McCain stuck to his guns, saying, "of course he said he wanted a timetable" for a withdrawal. McCain had made the allegation in Florida as he tried to shift the debate from the ailing economy, a stronger issue for Romney, a former venture capitalist and businessman.
Last April, Romney said U.S. and Iraqi leaders "have to have a series of timetables and milestones that they speak about" in private.
In Wednesday's debate, Romney said he was not calling for a specific withdrawal date. "It's simply wrong, and the senator knows it," he said.
For 90 minutes, Romney and McCain sharply challenged each other's conservative credentials and ability to lead the country. But they generally remained civil, and each called the other "a fine man."
Romney tried to portray McCain, who performs well among political independents, as out of the conservative mainstream as the contest moves toward a cluster of states where only registered Republicans can vote. He said the Arizona senator twice voted against President Bush's tax cuts and pushed campaign finance reforms that restricted fundraising and spending. The Republican establishment embraced the tax cuts and opposed the new campaign law, which many saw as helpful to Democrats.
"Those views are outside the view of mainstream Republican thought," Romney said. He made similar arguments in Florida, but lost to McCain by 5 percentage points.
McCain disputed the claims. "I'm proud of my conservative record," he said.
In a counterpunch, he said Romney left Massachusetts with high taxes and a large debt. "His job creation was the third worst in the country," McCain said, a claim Romney rejected.
The debate allowed McCain and Romney to focus on one another after Florida voters left no doubt that they are the party's two viable contenders. Former New York City Mayor Rudy Giuliani left the race earlier Wednesday and endorsed McCain.
During the debate, The Associated Press reported that California Gov. Arnold Schwarzenegger would endorse McCain on Thursday. Schwarzenegger was in the audience, as was Nancy Reagan, widow of the former president.
Former Arkansas Gov. Mike Huckabee and Rep. Ron Paul of Texas also participated in the debate televised by CNN, but largely watched as the two front-runners, who were seated next to each other, trade barbs. Huckabee protested this isn't a two-man race.
"If you want to talk conservative credentials, let me get in on that," said Huckabee, who has won no contest since the Jan. 3 Iowa caucus.
Paul reiterated his criticisms of the Iraq war and U.S. monetary policies.
McCain tried to deflect questions on illegal immigration, a sore point with many Republicans who resented his push for a Senate bill, ultimately unsuccessful, that would have granted a path to legal status for millions of undocumented immigrants now in the country.
Asked if he would vote for his bill now, McCain replied, "it won't" come to a vote "because people want the borders secured first." He said he supports new efforts to prevent illegal crossings.
California is one of several states voting on Tuesday that has a large immigrant population.
Romney said McCain opposed Bush's first-term tax cuts because they were tilted largely toward the rich. But he defended the cuts, saying, "I believe in getting rates down. I think that builds our economy."
McCain said he opposes tax cuts that are not coupled with spending restraints. Republicans lost congressional seats in 2006 less because of the Iraq war than because of out-of-control spending that alienated conservatives, McCain said.