Texas Gov. Rick Perry said Wednesday that Mitt Romney changed his position on legalized abortion out of political convenience, one of the sharpest allegations leveled yet in the South Carolina GOP presidential primary.

Romney, who had supported legalized abortion while Massachusetts governor, says he changed his mind after weighing legislation that "would have created new embryos for the purpose of destroying them."

Perry, who is struggling to keep his presidential hopes alive, has criticized Romney before, but not always so pointedly. He told an anti-abortion forum in Greenville, S.C., that it's hard to understand how a public official could change his views on something as fundamental as abortion in his 50s, after decades to think about it.

"This is a decision that Gov. Romney made for political convenience, not an issue of his heart," Perry said.

Perry has shifted his own views on abortion somewhat. He recently said he no longer supports legal abortions in cases of rape and incest.

"If you're going to be pro-life," he said Wednesday, "then you've got to be pro-life all the way."

Presidential contenders Newt Gingrich and Rick Santorum also spoke at the forum, and Ron Paul spoke by video feed from Washington. All three said human life begins at conception.

Romney declined to attend the "Presidential Pro-life Forum."

Gingrich, the former House speaker, said, "we are fully human upon conception" and that all constitutional rights "should attach at that moment."

Santorum, a former senator from Pennsylvania, said he doesn't merely believe human life begins at conception, he knows it. "It's a biological fact," he said.

Paul, a Texas congressman, said he delivered thousands of babies during his years in medical practice. The invention of the ultra-sound, he said, made it far easier to show pregnant women the human life growing inside them.