Mitt Romney compared rival John McCain with late President Richard Nixon Thursday, accusing McCain of persistently misrepresenting Romney's position on the Iraq war for political gain in a style "reminiscent of the Nixon era."

Romney's sharp criticism spilled over from the GOP debate in California the night before, where he said McCain was resorting to old-style "dirty tricks" by claiming Romney supported a timetable for troop withdrawal.

McCain leveled the troop-withdrawal charge the weekend before Tuesday's Florida primary, which he won, and repeated it at the debate. But Romney has persistently said he does not support a timetable for withdrawal.

"I think he took a sharp detour off the straight-talk express by stooping to the attack he did, and then of course by him continuing to say it is a very strange thing. And as I mentioned last night, every independent voice that I've seen has pointed out that what he's done is disingenuous and not honorable," Romney said in Long Beach, Calif., Thursday.

"Had he a question about this he could have raised it any time between April and now, but to raise it outside a debate and to do it ... to people in Florida was something reminiscent of the Nixon era and I don't think I want to see our party go back to that kind of campaigning. "

Romney is locked in a taut contest with McCain, and is trying to derail the Arizona senator's mounting momentum off his wins in Florida, South Carolina and New Hampshire.

Romney's campaign announced Thursday it would be going on air in several major states voting Feb. 5, when 21 states hold GOP contests.

Romney has tried to portray McCain as out of step with mainstream conservatives, and repeated the claim Thursday.

"I would say that Senator McCain has, over his career in Washington, demonstrated in very remarkable ways strong leadership, which has tended to be his most notable accomplishments, leadership ... for liberal causes," he said, citing his immigration reform package and other legislation. "I'm not calling him a liberal, but I'm saying those particular accomplishments were liberal."

He said at a stop in Fountain Valley, Calif., that this is a " battle for the heart of the Republican Party."

At Wednesday's debate, Romney blasted McCain's conservative credentials and said that by repeating the charge about troop withdrawal his rival was turning to "dirty tricks that I think Ronald Reagan would have found reprehensible."

The debate was held at the Ronald Reagan Presidential Library in Simi Valley, Calif., where the site's namesake was frequently invoked.

"I have never, ever supported a specific timetable" for withdrawing troops, Romney said.

McCain stuck to his guns, saying, "of course he said he wanted a timetable" for a withdrawal. Last April, Romney said U.S. and Iraqi leaders "have to have a series of timetables and milestones that they speak about" in private.

But Romney said he was not calling for a specific withdrawal date.

"It's simply wrong, and the senator knows it," he said. "I will not pull our troops out until we have brought success in Iraq."

FOX News' Shushannah Walshe and The Associated Press contributed to this report.