Fred Thompson, the GOP presidential candidate who has struggled to stay competitive and hold on to his second-place slot in national polls, could make inroads with conservative voters after fielding the endorsement Tuesday of the largest anti-abortion group in the country.

The National Right to Life Committee announced its backing for Thompson in Washington, D.C.,

“Our endorsement is a testament to Sen. Thompson's longstanding pro-life record, his commitment to protecting unborn children and our belief in his ability to win,” said Wanda Franz, president of the committee.

The group also cast him as the candidate most likely to beat abortion-rights supporter and frontrunner Rudy Giuliani in the primary race for the GOP nomination.

"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," said the group's executive director David N. O'Steen.

By emphasizing Thompson's political potential, the anti-abortion group played down its own differences with the former Tennessee senator. Thompson has been at odds with the group because he doesn't support a federal constitutional amendment outlawing abortion, a long-standing party platform plank; because he has called the Terri Schiavo right-to-die case a family matter; and because he backed campaign finance regulations that the group considers a restriction of free speech.

But in endorsing Thompson, the group cast their vote in the contest among candidates to be seen as the true conservative.

That's the battle among the GOP right now, and the candidates are playing it fiercely. Former Massachusetts Gov. Mitt Romney recently earned the backing of Moral Majority Co-Founder Paul Weyrich. Speaking in in Sioux City, Iowa, on Tuesday, Romney expressed disappointment he didn't receive the committee's endorsement.

"You always like all the endorsements you can get, but you can't get them all, and I'm pleased with the ones I've received and hope to keep getting more," Romney said.

Other candidates are also vying for conservative credentials. Arizona Sen. John McCain last week got the support of conservative Republican Sam Brownback, who dropped out of the presidential race last month. Giuliani last week received the heavy-hitting endorsement of evangelical leader Pat Robertson, despite his moderate views on issues like abortion and gay rights.

The National Right to Life Committee endorsement may come with more perks for Thompson. The committee is now poised to lend the candidate support in the form of direct mailings, advertisements and the ability to move votes.

"I'm deeply appreciative for the past support by the National Right to Life Committee-PAC in my Senate campaigns, and today I am blessed and grateful to have received their endorsement for President of the United States," Thompson said in a statement. "In supporting me, those who have worked tirelessly to defend life are supporting a consistent conservative who has stood with them yesterday, who stands with them today, and will stand with them tomorrow."

Meanwhile, Thompson spoke at The Citadel military college in South Carolina shortly after the endorsement was announced, where he laid out his plan to expand the military.

Thompson said he wants to boost the military's ground force of Army and Marines troops to 1 million members, about 250,000 more than the Pentagon has recommended.

He also talked about spending more on defense, modernizing the military and providing service men and women with better pay and benefits, including health care.

Thompson is polling strong in South Carolina, which holds its Republican primary Jan. 19. Recent surveys show him in a dead heat in the state with Giuliani and Romney, each polling at between 18 and 19 percent. But nationally, Thompson's support has gradually dipped since announcing his candidacy in September.

Rasmussen polls, each of about 600 likely Republican voters, taken between Nov. 8-11 showed Thompson with 14 percent support, compared with the 19 percent support he had less than a month ago in similar polls.

FOX News' Carl Cameron and Shushannah Walshe and The Associated Press contributed to this report.