GOP presidential candidate Newt Gingrich took a subtle swipe at the field's front-runner, Mitt Romney, on Tuesday for not signing an anti-abortion pledge. 

Gingrich (R-Ga.), a former House Speaker, said that Romney had made "excuses" for not signing the pledge, which was circulated to candidates by the anti-abortion-rights group Susan B. Anthony (SBA) List. 

The former Speaker told conservative talk-radio host Laura Ingraham that "people have to make that decision on [their] own" when it comes to reacting to Romney's handling of the pledge. While the former Massachusetts governor might have "his excuses" for not signing it, Gingrich said, "I thought it was a pretty clear-cut pledge."

Romney has taken heat over the last several days for not signing the SBA List pledge, which debuted at the Republican Leadership Conference in New Orleans last weekend. 

Gingrich, Rep. Michele BachmannMichele Marie BachmannYes, condemn Roseanne, but ignoring others is true hypocrisy Bachmann won't run for Franken's Senate seat because she did not hear a 'call from God' Billboard from ‘God’ tells Michele Bachmann not to run for Senate MORE (R-Minn.), former Sen. Rick Santorum (R-Pa.), Rep. Ron Paul (R-Texas) and former Minnesota Gov. Tim Pawlenty (R) are 2012 candidates who have signed the pledge.

Abortion rights are a politically sensitive area for Romney since he ran as a pro-abortion-rights candidate for governor in 2002. Romney explained his decision not to sign the pledge, saying its language is "overly broad and would have unintended consequences."

But that did not stop the group and his fellow presidential contenders from questioning his conservative credentials on abortion.

"It is distressing that Governor Romney refuses to sign the SBA Pledge, even while claiming to be pro-life," said Alice Stewart, a spokeswoman for Bachmann's presidential campaign. "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."

Santorum said Romney's decision was "incredibly disappointing."