Republican presidential contender Fred Thompson criticized Republican rival Rudy Giuliani, likening him to a liberal and questioning the former New York mayor's loyalty to the Republican Party.

"Some think the way to beat the Democrats in November is to be more like them. I could not disagree more," the one-time Tennessee senator says in remarks he was delivering Monday to the Conservative Party of New York. "I believe that conservatives beat liberals only when we challenge their outdated positions, not embrace them. This is not a time for philosophical flexibility, it is a time to stand up for what we believe in."

Thompson does not mention Giuliani in excerpts made available to The Associated Press, but he is clearly trying to draw a contrast with the rival leads in national Republican polls. Unlike Thompson, Giuliani backs abortion and gay rights. And, the ex-mayor's central argument for Republicans to nominate him is that he has the best chance to win in the general election.

Thompson was more direct in an interview on Fox News Channel before the speech.

"I don't think that the mayor has ever claimed to be a conservative," he said before rattling off Giuliani's history.

He noted that Giuliani, who ran as a Republican, sought and won the Liberal Party's endorsement in his first mayoral race in 1989, and that as mayor, he broke from the Republican Party and endorsed Democratic Governor Mario Cuomo in an unsuccessful race for a fourth term.

"So I don't know that he's ever claimed to be anything else," Thompson said, suggesting Giuliani was more a liberal than a conservative.

In response, Giuliani spokeswoman Katie Levinson said: "Fred Thompson can talk about labels all he wants, but labels are meaningless without results. Mayor Giuliani has done more than just talk like a conservative, he's governed like one and has the record to prove it."

Earlier, Giuliani's campaign arranged for several deputy mayors who served in his administrations to hold a news conference in Times Square to promote his success in reducing crime, overhauling welfare and cutting taxes. The message — Giuliani has showed results the conservative way — was meant to contrast with Thompson, who lacks executive experience.

"Some candidates talk the talk about Republican principles. Others actually have a proven track record of governing according to Republican principles," said Randy Mastro, a deputy mayor in Giuliani's first term.

With voting beginning in under three months, Thompson is trying to win the support of conservatives who are pivotal in Republican primaries.

"With me, what you see is what you get. I was a proud conservative yesterday, I remain one today, and I will be one tomorrow," Thompson says in the speech.

He touts his eight-year Senate tenure and boasts of working to further the conservative causes of smaller government, lower taxes, less regulation and conservative judges.

In fact, while he was seen as a reliably conservative vote in the Senate, he sometimes strayed from the party line and focused more on investigating than legislating.

Giuliani, for his part, was once a Democrat, voting for Democratic presidential nominee George McGovern in 1972. In his own two terms as mayor, Giuliani staunchly supported abortion rights, gay rights and gun control — and also was left-of-center on a host of other issues.

Thompson's address to the Conservative Party will be his first public event since participating in his first presidential debate in Michigan last Tuesday. He was scheduled to be in New Hampshire late last week for a fundraising breakfast for Manchester Mayor Frank Guinta, but he canceled the trip. Aides say he also plans stops this week in Washington, Georgia and Florida.