"Advance."

That's the one word GOP presidential candidate Herman Cain points to in response to conservative critics of his decision not to sign a pledge that some pro-lifers consider the test of whether candidates are truly one of them.

"They wanted me to say that I would advance that particular legislation. The president can't advance legislation, and when I challenged them on it, they didn't want to change it. I don't want to sign a pledge to say that I can do something that I cannot do," Cain told Fox News in an exclusive interview Sunday.

At issue is the Susan B. Anthony List 2012 Pro-life Pledge. And Cain isn't the only GOP contender to refuse to sign it.

The Susan B. Anthony List says former Massachusetts Gov. Mitt Romney, newcomer Jon Huntsman and former New Mexico Gov. Gary Johnson have also declined. But since Johnson has already identified himself as being pro-choice -- it's Romney and Huntsman who have taken a lot of the heat.

When asked why Cain hasn't gotten the same degree of blowback that GOP frontrunner Romney has gotten, Cain argues he explained his reasons for withholding his signature better than Romney did.

Cain said in a june 18 press release that he supports the majority of the pledge, but objects to the clause that requires he "advance" the Pain-Capable Unborn Child Protection Act. He goes on to say that he would sign the pledge if it made it to his desk in the Oval Office.

Cain tells Fox News that anti-abortion advocates "know exactly where I stand on this issue; it's not a question mark."

Shortly after Romney declined to sign the pro-life pledge, he penned an op-ed in the National Review Online explaining his decision.

"I am pro-life and believe that abortion should be limited to only instances of rape, incest, or to save the life of the mother... As much as I share the goals of the Susan B. Anthony List, its well-meaning pledge is overly broad and would have unintended consequences," Romney writes.

The former governor claims some hospitals would lose federal funding and argues the president's hand would be tied to the pledge, preventing him from appointing "the most qualified individuals."

Texas Rep. Ron Paul, former Minnesota Gov. Tim Pawlenty and former Pennsylvania Sen. Rick Santorum have all signed the pledge.

Minnesota Rep. Michele Bachmann also signed the pledge, and she chastised Romney in a June 26 appearance on "Fox News Sunday."

According to the Susan B. Anthony List, candidates who sign the pledge promise to only nominate judges to the Supreme and federal Courts who are committed to the original meaning of the Constitution' select pro-lifers for relevant cabinet and executive branch positions; advance legislation to permanently end all taxpayer funding for abortion, defund Planned Parenthood and all other groups affiliated with funding or performing abortions; and advance and sign Pain-Capable Unborn Child Protection Act.

Fox News Intern Adrienne DiPiazza contributed to this story.