McCain had eased up on his criticism of the former Massachusetts governor since winning the primary in Florida last week and had begun acting like a general election candidate, focusing his harsh rhetoric on Democratic rivals Hillary Rodham Clinton and Barack Obama.
But with hours to go before voting began in more than 20 states from coast to coast, McCain unloaded on Romney.
The 30-second ad airing on national cable television shows Romney distancing himself from Ronald Reagan, patron saint of modern conservatism, in a 1994 debate when he was challenging Democratic Sen. Edward M. Kennedy of Massachusetts.
"Look, I was an independent during the time of Reagan-Bush. I'm not trying to return to Reagan-Bush," Romney says in the ad footage. An announcer then intones, "If we can't trust Mitt Romney on Ronald Reagan, how can we trust him to lead America?"
McCain also ratcheted up the rhetoric during an interview with the CBS Evening News. Asked about his opponents' weaknesses, the Arizona senator reverted to the caricature of Romney as a flip-flopper. "He's had literally at least two positions on every major issue," McCain said.
Kevin Madden, Romney's campaign spokesman, noted the tension, saying McCain "has a long history of being an agitator" within the Republican Party.
"Governor Romney has a proven record of supporting core Republican principles like tax cuts and a strong border security policy," Madden said. "John McCain would be unable to stand strong against the Democrats on taxes and illegal immigration since he has voted with the Democrats so many times on those issues."
Romney also countered with automated telephone calls, including one recorded by former Sen. Rick Santorum of Pennsylvania and placed to thousands of Republican homes in Arizona, presumably including McCain's.
"As a conservative I don't agree with McCain on many issues and I don't think he has the temperament and leadership ability to move the country in the right direction," Santorum said in the recording.
The dispute between McCain and Romney is important because both are vulnerable on the question of how conservative they really are.
Any doubts are especially acute for McCain, who emerged from Florida positioned to become the GOP front-runner if he can win enough convention delegates on Super Tuesday.
Many conservatives in the GOP base don't trust McCain, and some like-minded talk show pundits insist he would destroy the party if he becomes the nominee. McCain's longtime friend Bob Dole, the 1996 Republican presidential nominee, defended McCain in a letter Monday to Rush Limbaugh.
As McCain assailed Romney publicly, the senator and his conservative surrogates were quietly reaching out to conservative leaders not yet on board with his candidacy, according to two GOP strategists with ties to the party's right flank. Those who aren't backing McCain have indicated they are most concerned about his opposition in 2001 and 2003 to President Bush's tax cuts and his work with Democrats to avoid a Senate showdown over Bush's judicial nominees.
The list of grievances against McCain is lengthy, including his sponsorship of legislation to limit money in politics. Critics say that violates free speech. Conservatives also are unhappy with McCain's support of a path to citizenship for millions of illegal immigrants.
Yet Romney is not the consensus candidate of frustrated conservatives, either. Two other contenders, former Arkansas Gov. Mike Huckabee and Rep. Ron Paul, have attracted only limited support, and some conservatives are looking for something else entirely.
One such activist, Virginia direct-mail consultant Richard Viguerie, on Monday called on conservatives to bring in a new candidate who can unite the party's various factions.
--- Associated Press writer Liz Sidoti in Washington contributed to this report.
