Just days after Rick Santorum said he regretted endorsing former Senator Arlen Specter's 1996 bid for the presidential nomination, Specter has hit back, saying Santorum is “too far out of the mainstream.”

Specter, a former Republican, endorsed the fellow Pennsylvanian's 1994 Senate campaign. Specter switched to the Democratic Party in 2009 and has since come out in favor of gay marriage, reversing his previous opposition.

But last week, Santorum said he regretted his decision.

“I certainly knew that Arlen Specter was going nowhere and I certainly disagreed with a lot of things that he said,” Santorum said Sunday on ABC's This Week. “It was something I look back and wish I hadn't done.”

“That certainly wasn't one of my prouder moments that I look back on,” he added.

During an appearance Tuesday on MSNBC's Hardball, Specter swiped back at Santorum, saying, “When rick takes position that women don't belong in the workplace. When he takes positions that the gay issue is bestiality – a man on dog – and the contraception ought not to be followed, he is so far out of the mainstream that I think even the Republican Party won't take it.” (The video is embedded in the right panel of this page. Visit our video library for more videos.)