Perhaps one of the most stinging comments during Monday night's GOP presidential debate in Myrtle Beach, S.C., came from a candidate who wasn't on the stage.

Former Utah Gov. Jon Huntsman dropped out of the race on Monday, but his earlier jabs at current GOP frontrunner Mitt Romney made an appearance at the Fox News forum: A debate moderator confronted Romney with a Huntsman complaint that Romney is "a perfectly lubricated weather vane on the issues of the day."

If the jab hurt, Romney didn't show it. Indeed, the punch carried an irony that softened it: When Huntsman left the race on Monday he endorsed Romney. (See "Huntsman out," by Edward Lee Pitts, Jan. 16.)

That reality captured the dynamic at the Myrtle Beach debate ahead of South Carolina's GOP primary on Saturday: Though Romney's opponents landed a few punches, they didn't achieve fatal blows. If that pattern continues for the next few days, including at another nationally televised debate in Charleston on Thursday, it may be difficult for any one of them to break away from the pack and mount a serious challenge to the former Massachusetts governor in a state that rejected him in 2008.

Consider the punches: One of the testier moments of Monday's debate came when Texas Gov. Rick Perry demanded that Romney release his federal tax returns. A tongue-tied Romney looked clearly uncomfortable as he dodged the question, but he later suggested he might release the records. Even if that exchange scored points for Perry, it isn't likely enough to pull him from his current position in last place.

Another punch: Rick Santorum flummoxed Romney when he asked him if felons should be allowed to vote once they served their sentences. Romney searched for an answer before settling on one: Violent criminals shouldn't be allowed to vote again. Santorum quickly pointed out that Massachusetts laws during Romney's tenure as governor allowed convicted felons to vote while still on parole. But if that punch scored debate points for former senator from Pennsylvania, it likely wasn't an issue that resonates strongly with South Carolina voters.

Other candidates managed to score points for some of their positions but struggled to defend others: Newt Gingrich won loud applause for insisting that unemployment benefits should come with time limits and expectations that the unemployed are seeking jobs and training. But the former House Speaker struggled to defend his most pointed line of attack against Romney's tenure as a manager at Bain Capital.

Ron Paul won praise for insisting on cutting spending and ending wars, but he drew sustained boos from the crowd for suggesting that American authorities mishandled the capture of terror mastermind Osama bin Laden. When the Texas congressman said America wouldn't approve of China swooping in to kill a Chinese dissident on American soil, Gingrich retorted, "Osama bin Laden isn't a Chinese dissident. It's an absurd analogy."

While Romney remained above the fray during some of his opponents' in-fighting, a question from a voter via Twitter reminded the audience of one of the candidate's biggest challenges. The voter noted how many times Romney had changed his mind on important issues and implored: "Convince me that you won't change your mind again."

Romney acknowledged that the voter was likely talking about his transition from pro-abortion to pro-life. The candidate briefly explained when he had changed his mind on the issue, adding that he remains pro-life now. Romney also said he has always opposed same-sex marriage. But his comments on social issues quickly morphed into a criticism of President Barack Obama, and a warning that the president wants to make the country an entitlement state.

The strategy to address social issues quickly but sparsely remains a hallmark of Romney's campaign, though the candidate did attend a forum on Monday sponsored by the South Carolina Faith and Freedom Coalition. Romney told the crowd of evangelicals that "the next president of the United States should stand up for the sanctity of human life in this country, and anywhere in the world where it is threatened." He also repeated his support for a constitutional amendment to defend traditional marriage.

But if voters want to ask Romney direct questions about his pro-life position, there's one place they won't be able to do it: A pro-life forum with all the other candidates on Wednesday night.

Personhood USA-a pro-life Christian ministry based in Colorado-invited the GOP presidential candidates to participate in Wednesday's pro-life forum in Greenville, S.C. The 90-minute event will offer candidates an opportunity to answer voters' questions about pro-life issues. All of the remaining Republican presidential candidates plan to attend, except Romney.