Republican presidential candidate Mitt Romney on Friday told hundreds of anti-abortion activists that his conversion to their cause is genuine as he sought to fend off rivals' criticism that he's inconsistent on the issue.

"I know that it is not time but conviction that unites us," Romney said in remarks on the second day of the National Right to Life's annual convention. "I proudly follow a long line of converts — George Herbert Walker Bush, Henry Hyde, and Ronald Reagan to name a few."

Romney's speech was interrupted several times by applause.

"My experience as governor taught me firsthand that the threat to our culture is real," Romney said. "When responsibility for life or ending life was placed in my hands, I made the right decision."

Two of Romney's rivals — Sens. John McCain of Arizona and Sam Brownback of Kansas — have questioned the former Massachusetts governor's record on abortion. Romney repeatedly vowed not to change state abortion laws and backed abortion rights as recently as 2 1/2 years ago, even though he insists he has always personally opposed the practice.

Now, as a presidential candidate, he not only emphasizes his personal opposition to abortion rights, but he also calls for the repeal of Roe v. Wade, the 1973 Supreme Court decision legalizing abortion nationally.

Brownback, who attended the convention, was asked about abortion opponents who back Romney. He responded: "I'd say, 'Look at me first.' I'm somebody who's been consistent."

The Kansas senator, a long shot for the nomination, said, "I've been fighting this fight for a long time and I believe in it and I think it's the central social, moral issue of our day."

Just this week, McCain's campaign circulated a video clip showing Romney reiterating his vow to uphold the state's abortion-rights laws in May 2005. The Romney campaign argued that he made the remark at a news conference in which he spoke of "respecting human life" as he vetoed state legislation that would expand embryonic stem cell research.

Referring to his conversion, Romney told the convention of the largest grass-roots, anti-abortion group: "I am evidence that your work, that your relentless campaign to promote the sanctity of human life, bears fruit."

Opposition to abortion rights is a critical issue for many GOP conservatives, who hold sway in primary voting. Rudy Giuliani, who leads in national polls, backs abortion rights though he personally opposes the practice.

Romney has the edge in surveys in several of the early states such as Iowa and New Hampshire. He has sought to win over conservatives, arguing that he is with anti-abortion activists.

"I am humbled to be standing among the many who have toiled for the pro-life movement for so long, when I arrived at this place of principle only a few years ago," Romney said.

Henry Potrykus, of Lindenhurst, Ill., said he is supporting Brownback, because he has been consistent on his anti-abortion views.

"Romney's been flip-flopping," he said. "He was pro-abortion."