GREENVILLE, S.C. -- As they quizzed four candidates on their positions on abortion, the moderators of a pro-life forum here gave each presidential hopeful an opportunity to take a jab at the one candidate who did not participate in the event: Mitt Romney.

One of the moderators at the event, sponsored by Personhood USA, told the crowd of at least 250 at the Greenville Hilton hotel that the Romney campaign said they had scheduling difficulties, but that a similar scheduling conflict arose during a similar event in Iowa.

Underscoring his absence, candidates were each asked to contrast their positions with those of Romney, or asked what they would handle certain situations differently than he did as Massachusetts governor.

"How would you differentiate your record and Gov. Romney's record on life?" one of the panelists at the forum, sponsored by Personhood USA, asked Rick Perry, the first of the candidates to speak.

"We don't have enough time," Perry said as the crowd laughed.

He said he took issue with Romney's switch from favoring abortion rights to being pro-life in his fifties, saying he would not have such a problem with it if his revelation had occurred earlier in life.

"In your fifties…  it was clear to most of us that this was a choice for convenience. This was a decision that Gov. Romney made for political convenience, not an issue of his heart."

Newt Gingrich was asked to address Romney's signing of 198 same-sex marriage licenses, which Romney said he was compelled to do by the state supreme court.
 
"Can a state or federal executive challenge a constitutional ruling when they believe it is clearly erroneous?" one of the moderators asked.

Gingrich began his answer not by talking about the marriage licenses, but by criticizing both Romney's switch on abortion and his signing into law the Massachusetts health care plan, which includes an individual mandate to buy insurance.

"Gov. Romney also, after becoming pro-life, passed Romneycare with taxpayer funded abortions," Gingrich said.
 
Continuing his attack against the former governor's shifting positions, Gingrich said "his administration approved paying for an abortion clinic for Planned Parenthood with state funds and he appointed pro-abortion and pro-gay marriage judges to the court. So in terms of who's likely to change Washington I would say that his track record has not only not changed but is in the opposite direction."

Both Rick Santorum's and Ron Paul's questions on Romney dealt at directly with the Massachusetts health care law.

The moderators linked Santorum's ultimate question - whether he would "lead the charge against Obamacare" - with Romney by saying that the Massachusetts plan formed a model for the national program; a seemingly tangential preamble.

"I wouldn't be in this race if it wasn't for Obamacare," Santorum said. "Obamacare makes every single American dependent on the federal government for health, for life… You have my assurance that we will repeal Obamacare."

And Paul's question was preface with an explanation about the seat permanently held by Planned Parenthood on the Massachusetts plan's payment policy advisory board.

"Would you use your appointment power as president to assure that pro-life advocates are represented in the federal government?" he was asked.

Paul said he would "absolutely" appoint such advocates if they were pro-liberty and believe in non-violence, which he said were the qualifying factors for being pro-life. But, he said, such a distinction would be unnecessary because there would be no advisory board similar to that in Massachusetts.

"This wouldn't be a problem because they're not going to get any money, we shouldn't even have one," Paul said.

DES MOINES, Iowa – With only 20 days until the Iowa caucuses, four GOP candidates made their pitch to social conservatives tonight at the premiere of an anti-abortion documentary narrated by the former Arkansas Gov. Mike Huckabee.

Michele Bachmann, Newt Gingrich, Rick Perry, and Rick Santorum addressed the 1200 person crowd before the house lights dimmed for the “The Gift of Life” premiere.

“I do want you to take note,” Huckabee told the crowd. “There were four candidates who cleared their schedules, and made this a priority event.”

Former Pennsylvania Sen. Rick Santorum, trailing in Iowa polls, won the biggest applause from the crowd tonight – and aimed his remarks at his competitors.

“I have some problems with some of the folks who running for office these days when they say, ‘I believe life begins at conception.’  That’s like, I say, ‘I believe the sun rises.’” Santorum said, to laughs.

“Why would you say you believe something that’s a fact?” Santorum added. It seemed to be a reference, at least in part, to Gingrich, who spoke minutes earlier in favor of a congressional bill that would define personhood as beginning at conception – though Santorum said later tonight he was talking about a number of his opponents. "I know that there have been several candidates for president who have stated they believe life begins at conception – and as I said, it’s not a belief, its a fact," Santorum told NBC News.

During her remarks, Minnesota Congresswoman Michele Bachmann attacked the Obama administration for considering – before reversing course – making the “Plan B” morning-after pill available on pharmacy shelves, “where little girls could find it next to bubble gum and next to M&M’s."

"President Obama is so tied up in his reelection that even he knew that was one step too far,” Bachmann said. Governor Rick Perry touted his record defunding Planned Parenthood in Texas, where he said 12 clinics have closed as a result. He called the new film a tactic in the fight against abortion, saying, “imagine the difference you can make not in just one life, but in two.”

Attacks on Gingrich awaited people after the movie premiere.  A group billed as "Iowans for Life" paid for fliers on cars that read, "The bottom line: Newt Gingrich is a pro-life fraud."

But as Huckabee pointed out during his short remarks inside the theater: “I think it is significant that all four of the candidates who are present tonight have endorsed life. And that ought to be very important.”