Perhaps the best evidence of Mitt Romney's new status as the clear frontrunner in the Republican field is the ferocity with which his competitors have begun attacking him.

Some of the other candidates for the Republican presidential  nomination were accused of being too genteel with their criticisms of the former Massachusetts governor during last week’s debate in Manchester, N.H., but they are making up for it this week with repeated broadsides against Romney.

The shots are mostly aimed at Romney's refusal to sign a pledge offered by pro-life group Susan B. Anthony List pro-life pledge.

The pledge calls on signers to support  only judicial nominees who are pro-life, to select pro-life cabinet members,  to stop taxpayer funding of abortions, and to support a bill that would  ban abortions after 20 weeks of pregnancy.

Five of the Republican presidential candidates, including Minnesota Rep. Michele Bachmann, former House Speaker Newt Gingrich,  Texas Rep. Ron Paul, former Pennsylvania Sen. Rick Santorum and former Minnesota Gov. Tim Pawlenty, have all signed the pledge.

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Businessman Herman Cain did not sign it, citing Congressional jurisdiction over abortion funding. But it was Romney’s refusal that set off the rest of the field.

"The excuses for not signing clearly continue the doubts about his leadership and commitment to ending the practice of abortion - particularly for a candidate who ran as pro-choice for the Senate and governorship of Massachusetts," says a statement from Bachmann’s campaign.

Santorum  was just as caustic.  "This past Monday night at the Republican Presidential Debate, I was asked about Governor Romney's pro-life conversion, and I gave him the benefit of the doubt.  I apparently spoke too soon," said Santorum.

Jim Talent, a former Republican Senator from Missouri, and one of Romney’s top advisers, defended the candidate's  refusal to sign the pledge, while touting Romney's pro-life credentials.

"He's strongly pro-life. He just affirmed it again, and it's on our website, a whole array of pro-life positions that went further, I think, than the pledge went," Talent told Fox News.

Talent went on to say  that Romney's decision was based  on what the campaign said are overly broad requirement of the pledge that would prohibit the consideration of any pro-choice appointee, even if that appointment is for a non-health-related position.

"He (Romney) doesn't want to say he's disqualified from considering people who may not agree with him on everything on that issue for a job like FBI director, " said Talent.

Former President George W. Bush adviser Brad Blakeman said he thought Romney was making the right move in refusing the influential group’s demands.

"You can't pander to third-party groups. If you pander to this group, what about the gun lobby, what about others who want you to take a tax pledge? " Blakeman told Fox News. “I think [the abortion pledge] is not something that is not going to be on the minds of Republican voters or the voters in November. It's going to be all about the economy."

But social conservatives are a core part of the Republican primary electorate and many are expressing excitement about the possible entry of pro-life Texas Gov. Rick Perry into the presidential field. Perry’s team says that the governor is weeks away from a decision, but in a series of speeches around the country testing the waters, Perry has expressed scorn for Republicans who equivocate on the issue of abortion.

''Our loudest opponents on the left are never going to like us, so let's quit trying to curry favor with them,” Perry said to cheers at the Republican Leadership Conference in New Orleans on Saturday.

To further complicate matters for Romney, former Utah governor and ambassador to China Jon Huntsman is set to announce his own presidential candidacy on Tuesday in a series of high-profile events. Huntsman, who has been shunned by some social conservatives for his support for civil unions for gay couples, has been ardently courting the moderate Republican on whom Romney has built his coalition.