ORLANDO, Florida (CNN) -- Republican presidential candidates intensified their fight over which one has the true conservative credentials in a sometimes contentious debate Sunday night.

GOP hopefuls, from left, Rudy Giuliani, Mitt Romney, Fred Thompson and John McCain take the stage Sunday.

The candidates also didn't miss the opportunity to criticize Democratic front-runner Sen. Hillary Clinton of New York.

A more aggressive Fred Thompson compared former New York Mayor Rudy Giuliani's social positions with Clinton and also implied former Massachusetts Gov. Mitt Romney once held more liberal stands than Sen. Ted Kennedy, D-Massachusetts.

Asked whether Romney and Giuliani had convinced him they were consistent conservatives, the former senator from Tennessee said, "Well, we've got an hour and a half. Maybe they can work on it."

Giuliani tried to turn the tables on Thompson, saying, "You know, Fred has his problems, too."

Giuliani said, "Fred was the single biggest obstacle to tort reform in the United States Senate. He stood with Democrats over and over again. Fred Thompson, along with very few Republicans, blocked tort reform over and over and over again. That is not a conservative position."

Defending his record a day after finishing near the bottom of a conservative voter straw poll, Giuliani said, "I think it was a good conservative record. I think, in every case, you can always find one exception or two to someone being absolutely conservative or absolutely this or absolutely that, but I think I had a heck of a lot of conservative results."

Asked if there was little difference between himself and Clinton on social issues, he replied, "You have got to be kidding."

Giuliani added, "She made a statement last week, and I've been very critical of her, but I want to tell her I agree with this one." He then quoted her as saying: " 'I have a million ideas; America cannot afford them all.' "

"No kidding, Hillary. American can't afford you."

Sen. John McCain of Arizona blasted Romney for claiming to be the only real Republican in the field.

"Gov. Romney, you've been spending the last year trying to fool people about your record," McCain said. "I don't want you to start fooling them about mine. I stand on my record. I stand on my record of a conservative, and I don't think you can fool the American people. I think the first thing you'd need is their respect."

McCain, a prisoner of war during the Vietnam War, drew laughs by criticizing an earmark that Clinton backed and the Senate stripped last week. "Sen. Clinton tried to spend $1 million on the Woodstock Concert Museum. Now, my friends, I wasn't there. I'm sure it was a cultural and pharmaceutical event. I was tied up at the time," he said.

Romney continued his criticism of Giuliani over spending and taxes, saying, "We're not going to keep Hillary Clinton out of the White House by acting like Hillary Clinton. Now I'm proud of my record. Not just of the words but of the record of the governor of Massachusetts," he said.

"Like Mayor Giuliani I had a tough state to be running in. I was a conservative Republican in a very Democrat state. My legislature, 85 percent Democrat. We face a $3 billion budget gap. We solved it without raising taxes, without adding debt."

Thompson joined the direct attack on Giuliani, criticizing his stand as mayor on immigration, abortion and gun control. He said Giuliani "sides with Hillary Clinton on each of those issues I just mentioned."

Asked about his concern over Romney's record, Thompson said. "You've just got a minute?"

After watching the top candidates in the Republican presidential field attack each other in the opening minutes, former Arkansas Gov. Mike Huckabee stayed above the fray.

Huckabee, left out of the early skirmishes between candidates running ahead of him in the polls, said as he was called on: " Let me begin by saying, for the first time in about nine debates, I'm kind of glad I wasn't in on the first few minutes because it was all about these guys fighting each other."

To laughter, Huckabee said, "I am more than content to let you let them fight all they want tonight, shed each other's blood, and then I'll be ready to run for president because I'm not interested in fighting these guys." E-mail to a friend

All About Mitt Romney • John McCain • Rudolph Giuliani • Fred Thompson