WASHINGTON (CNN) -- The National Right to Life Committee, a key anti-abortion group, endorsed Fred Thompson for president Tuesday, saying the former senator was the best candidate to beat Republican front-runner Rudy Giuliani.
Thompson trails Giuliani, a former New York mayor who supports abortion rights, in the race for the Republican presidential nomination, according to most national polls.
National Right to Life Executive Director David O'Steen suggested that Giuliani's lead played a part in the committee's decision to endorse Thompson.
"We look at the fact that while there are various polls, and some are up and down, the overwhelming consensus has been that he is best positioned to top pro-abortion candidate Rudy Giuliani for the Republican nomination," O'Steen said, "and also, looking at polls against the likely Democrats, he is well-positioned, and we believe best positioned, to win the presidency of the United States for unborn children."
Thompson did not attend the group's event announcing the endorsement at the National Press Club.
Karen Cross, the committee's political director, said the Tennessee Republican has had a consistent anti-abortion stance going back to his 1994 election to the Senate. Watch a pro-life boost for Thompson »
"Fred Thompson has shown by his consistent pro-life voting record in the U.S. Senate that he is a part of the pro-life majority," Cross said. "National Right to Life is confident in Sen. Fred Thompson's resolve to protect and defend the most defenseless among us: our unborn children."
Thompson left the Senate in 2003.
Thompson's anti-abortion credentials came into question in July when a former law colleague said the candidate lobbied for the National Family Planning and Reproductive Health Association in 1991.
Memos obtained by CNN at the time suggest the association had hired Thompson, but Thompson spokesman Mark Corallo said that the former lawmaker "has no recollection of doing any work on behalf of this group."
Darla St. Martin, National Right to Life's co-executive director, said she was not concerned about reports before his 1994 Senate campaign that he was not a staunch advocate of abortion rights.
"In 1994, I flew down to Tennessee and personally met with Fred Thompson," St. Martin said. "And I came away assured that he was pro-life.
"So, whatever this other rumor was, it was not true. I talked to him. He was pro-life. We then endorsed him. He was elected as pro- life. And during all of this term, he voted pro-life."
The announcement is a much needed boost for Thompson's campaign, which is suffering from low poll numbers and criticism the candidate lacks enough passion to win.
The National Right to Life Committee boasts affiliates in all 50 states with more than 3,000 local chapters nationwide. Some GOP strategists said the group's grass-roots power may not be as strong as it was in the past, but it will likely mobilize crucial support for Thompson's campaign.
Thompson came into the race late with the hope of winning over social conservatives unsatisfied with the rest of the GOP field.
Polls and anecdotal evidence suggest he's failed to excite those conservatives, but he is making a big push to play up his conservative credentials on the stump and on TV. Thompson is emphasizing his anti-abortion views and voting record in the Senate.
However, some conservatives have criticized Thompson for what they see as conflicting statements. Last week, he said he does not support an anti-abortion constitutional amendment -- a Republican platform position since Ronald Reagan. Thompson said he hopes Roe v. Wade will be overturned, but he said he believes abortion laws should be left to the states.
Republican presidential candidates are battling for support from the socially conservative wing of the party, often citing endorsements from leading evangelical figures.
In a surprise move last week, TV evangelist and Christian Coalition founder Pat Robertson endorsed Giuliani.
Robertson said he is endorsing Giuliani because he is "a proven leader who is not afraid of what lies ahead and who will cast a hopeful vision for all Americans."
The Giuliani campaign is counting on Robertson's endorsement to help him make inroads among evangelical Christians. Giuliani is trying to make the case with social conservative voters that he is an acceptable choice as the Republican nominee despite his support for abortion and gay rights.
Robertson repeatedly has praised Giuliani despite their differences on social policy. Both men have said a friendship developed after a long conversation on a plane trip to Israel several years ago. Both men also are survivors of prostate cancer.
Giuliani trails in surveys in many of the early primary and caucus states, among them Iowa and South Carolina, where socially conservative voters make up a major part of the Republican voting electorate.
The Robertson endorsement also may quash talk of a social conservative third party candidate if Giuliani wins the nomination.
It also put a dent in Mitt Romney's courting of the religious right. The former Massachusetts governor has made major inroads with Christian conservatives despite concerns about his Mormon faith.
Concerning Robertson's endorsement of Giuliani, Romney said, "You can't get them all." Romney has touted his recent endorsements from conservative activist Paul Weyrich and Bob Jones III, president of the evangelical Bob Jones University in Greenville, South Carolina.
Also last week, former White House hopeful Sen. Sam Brownback, a Kansas Republican who is the leading voice for Christian conservatives in the Senate, endorsed Sen. John McCain from Arizona.
Brownback gave up his bid for the White House last month after lackluster fundraising and poor showings in polls.
McCain opposes legalizing abortion and gay marriage, but he is not a darling of the far right. Brownback is a socially conservative senator who emphasized his opposition to abortion, gay marriage and other issues important to Christian conservatives.
His endorsement could help McCain in Iowa, where he trails badly in the polls to Romney. The Iowa caucuses kick off the presidential primary calendar January 3. E-mail to a friend
CNN's Dana Bash, John King and Mark Preston contributed to this report.