Pro-Santorum super-PAC buys ads in southern primary states

"On the heels of our call for Newt Gingrich to unite conservatives by exiting the race, now we are investing heavily in the next two primary states to ensure that happens with a Santorum victory,” Red, White and Blue Fund adviser Stuart Roy said in a statement. "Then voters will be able to have a choice between an authentic conservative in Santorum and a calculated conservative in Mitt Romney."

Gingrich's campaign on Wednesday said Alabama and Mississippi are must-wins for the House former Speaker.

The Red, White and Blue Fund argues Santorum would have a shot at beating Romney if Gingrich would withdraw from the race.  

"With Gingrich exiting the race it would be a true head-to-head race and conservatives would be able to make a choice between a consistent conservative in Rick Santorum or Mitt Romney," Roy said in a statement on Wednesday. "For instance, with Gingrich out of the race, Santorum would have won both Ohio and Michigan. Newt has become a hindrance to a conservative alternative."

Gingrich has rejected calls for him to drop out.

"We are staying in this race because I believe it is going to be impossible for a moderate to win the general election," Gingrich said at a campaign event in Alabama, according to the Atlanta Journal-Constitution.

Watch the super-PAC’s healthcare ad here.

In response to the ad, Romney's campaign accused the Santorum campaign of twisting and exaggerating the former Massachusetts governor's record..

"Sen. Santorum and his allies have a habit of making distortions, exaggerations and falsehoods about Mitt Romney’s record while at the same time denying Sen. Santorum’s well-documented lobbying past," Romney spokeswoman Andrea Saul said in a statement. "Voters are tired of the same political double-speak and understand it’s going to take an outsider to change the status quo in Washington."

—This story was updated at 12:52 p.m.