Former Sen. Rick Santorum (R-Pa.) quickly attacked Mitt Romney on Thursday after the former governor delivered a major speech in which he offered no apologies for the healthcare law he signed in Massachusetts.

Speaking in Michigan Thursday, Romney said it would be dishonest for him to repudiate the plan that he called "right for the people of my state."   

Santorum, a likely Romney rival for next year's GOP presidential nomination, wasn't impressed. 

"I greatly respect Governor Romney and admire many of his personal and professional accomplishments, but his work to institute the precursor to national socialized medicine is not one of them," Santorum said in a statement. 

The former senator, who was a Romney backer in 2008, has been the most willing among the party's likely 2012 contenders to go after Romney for the healthcare plan he championed in Massachusetts.

"Both RomneyCare and ObamaCare infringe upon individual freedom and exponentially increase the government's healthcare cost burden," Santorum continued. "RomneyCare has, in fact, not made healthcare better or saved costs in Massachusetts. It's done just the opposite. 

"This is not a failure of execution, but a lack of foresight on Governor Romney's part to understand the implications of his policy proposals. We need leaders who believe in the American people again, not the power of government to solve our problems. Yes, the Governor had the right to implement Romney-ObamaCare at the state-level, but that does not make it the right thing to do," Santorum said.

At this point, other likely Republican presidential contenders have declined to attack Romney directly over the Massachusetts healthcare law.

In defending his plan Thursday, Romney rebutted conservative critics, arguing his plan was very different from President Obama's.