"You've just spent the last year trying to fool people about your record. I don't want you to start fooling them about mine," Arizona Sen. bluntly told Romney, the former governor of Massachusetts.
Former Sen. made Giuliani his target, saying the former New York mayor supported federal funding for abortion, gun control and havens for illegal immigrants.
"He sides with on each of those issues," added Thompson, referring to the New York Democrat who leads in the polls for her party's presidential nomination.
The clashes in the early moments of a 90-minute debate prompted former Arkansas Gov. to say he wanted no part of a "demolition derby" with others of his own party. "What I'm interested in is fighting for the American people."
"Time is growing short in this nomination fight," said CBSNews.com senior political editor Vaughn Ververs. "We're headed for a battle which will be won in the trenches and none of these Republicans have been able to gain the upper hand."
The debate was the first since Sen. of Kansas dropped out of the race, winnowing the field. The remaining rivals stood on a stage at a resort 10 miles from Walt Disney World, fielding questions at an event broadcast by Fox News Channel.
The leadoff Iowa caucuses are scheduled for Jan. 3, 2008, for Republicans. In their most recent debate, Oct. 9, Giuliani and Romney swapped charges with each other, vying for primacy in the race.
This time they largely ignored each other. Instead, Giuliani's lead in the nation polls, as well as Romney's perceived strength in early voting states, made them obvious targets for McCain and Thompson.
The first question went to Giuliani, asked whether he was more conservative than Thompson. "I can't comment on Fred," the former mayor said.
He then added that he had brought down crime, cleaned up Times Square, cut taxes and eliminated the city's deficits. "I think that was a pretty darned good conservative record," he said.
Giuliani took a more conservative position on gay marriage than he has thus far, saying he would support a constitutional amendment banning gay marriage if states begin to legalize it.
Giuliani lived with an openly gay couple after separating from his second wife, Donna Hanover, and one member of the couple said at the time that Giuliani promised to marry them if gay marriage was ever legalized.
Attacked by the former Tennessee senator moments later, Giuliani fired back at his antagonist. "Fred has problems, too," he said. He said Thompson was the "single biggest obstacle" in the Senate to legislation limiting the ability of individuals filing lawsuits to recover unlimited damages.
"He stood with the Democrats over and over again" on the issue, Giuliani added.
Thompson said he believed states should decide whether to limit lawsuits in their own states.
Republicans in Congress tried for years to pass legislation that would cap damages in lawsuits, but never succeeded before losing their majority to Democrats in 2006.
Romney was asked about McCain's earlier claims that he had shifted positions on a number of issues to appeal to conservative Republicans.
The former Massachusetts governor responded that he was proud of his record, particularly since the state had an overwhelmingly Democratic Legislature. "I fought to make sure we kept our taxes down. I fought for pro-growth strategies. I cut taxes," he said.
Moments later, though, McCain personally turned on Romney.
"Governor Romney, you've been spending the last year trying to fool people about your record. I don't want you to start fooling them about mine," he said.
Saying he would run on his record as a conservative, McCain added, "I don't think you can fool the American people. I think the first thing you'd need is their respect."
Whatever their disagreements among one another, the eight rivals agreed on one issue. They took turns criticizing Clinton, the Democratic front-runner.
Asked whether she was fit to be commander in chief, Romney replied, "I'd vote no."
Giuliani said he agreed with one thing the former first lady said recently. "I have a million ideas America. Cannot afford them all," he quoted her as saying as laughter filled the debate hall. "I'm not making it up."
McCain said Clinton had recently tried to spend $1 million on a Woodstock Museum, commemorating perhaps the most famous counterculture event of the 1960s.
"Now my friends I wasn't there. I'm sure it was a cultural and pharmaceutical event," he said.
"I was tied up at the time," he deadpanned, and the audience rose to applaud the reference to the five and a half years McCain spent as a prisoner of war during Vietnam.