"The reason we have ObamaCare is because the senator you supported over Pat Toomey in Pennsylvania, Arlen Specter … he voted for ObamaCare. If you had not supported him, if you had not supported Arlen Specter, we would not have ObamaCare, so take a look in the mirror," Romney said.

Santorum defended the endorsement of Specter — who eventually switched from a Republican to a Democrat and voted in favor of the president's healthcare law — saying he had secured a guarantee that Specter, who then chaired the Senate Judiciary Committee, would back Bush's Supreme Court nominees.

"Every nominee he supported passed because he gave Democrats cover to vote for him and moderate Republicans cover to vote for him … I did the right thing for our country," Santorum said.

Specter said Thursday that he thought Santorum's endorsement made little difference in the primary race against then-Rep. Pat Toomey (R-Pa.) Toomey went on to challenge Specter again in 2010 when the incumbent was running as a Democrat, eventually winning the seat.

"His support was hardly determinative, hardly that important," Specter said, noting his campaign ran ads featuring endorsements from Bush rather than Santorum.

Specter's comments were first reported in the Washington Post.