Washington, DC (LifeNews.com) -- The Republican presidential candidates had one of their more contentious debates on Sunday as they debated which of the hopefuls is more in tune with the beliefs of pro-life advocates. With one of the more stalwart pro-life Republicans in Sam Brownback dropping out of the race, remaining candidates sought his pro-life mantle.

During the debate, Fred Thompson, the former Tennessee senator, challenged Rudy Giuliani's claims that he is a conservative, citing the former New York mayor's support for abortion.

“Mayor Giuliani believes in federal funding for abortion,” Thompson said while naming off several issues he thinks would turn off Republican voters.

Thompson faced a question on paid lobbying he did more than 15 years ago when he worked for a law firm that had a pro-abortion group as a client.

He pointed out his pro-life voting record while he was in the Senate -- "All that time, I compiled a 100 percent pro-life voting record."

Thompson said the work was for "my private law practice, as opposed to my public service."

"I was a member of a firm, of counsel to a large firm. And it was their client. They asked me to do a little work on it. I made a few calls. And that was that," he said.

"Frankly, I'd forgotten about it. But they've come forward now, because I'm their worst nightmare," he said of abortion advocates. "After that happened, I went to the United States Senate and voted consistently against them on every bill that came up. Now they're trying to defeat me."

Meanwhile, Arizona Sen. John McCain criticized former Massachusetts governor Mitt Romney for his shift from supporting to opposing abortion.

“Governor 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.”

But Romney shot back with a blast at Giuliani's pro-abortion stance, saying "We're not going to keep Hillary Clinton out of the White House by acting like Hillary Clinton."

Former Arkansas Governor Mike Huckabee also got in the middle of the debate on abortion, and he laid out his pro-life views.

"I do believe that it is one of the defining issues of our culture and civilization in that it expresses our understanding that every single human being in this society has intrinsic value and worth," he said about the sanctity of human life.

"When our founding fathers put their signatures on the Declaration of Independence, those 56 brave people, most of whom, by the way, were clergymen, they said that we have certain inalienable rights given to us by our Creator, and among these life, liberty and the pursuit of happiness, life being one of them. I still believe that," he added.