WASHINGTON -- Potential Republican presidential hopeful Mitt Romney rejected conservative critics who say he should apologize for approving a government mandate to obtain health insurance when he was governor of Massachusetts. Romney defended his actions in a Thursday speech to doctors in Michigan.

"A lot of pundits around the nation are saying I should stand up and just say, 'This whole thing was a mistake,'" Romney said, speaking at the University of Michigan Cardiovascular Center. "I presume a lot of folks would conclude that if I did that that would be good for me politically."

"But there's only one problem with that. It wouldn't be honest," Romney said. He admitted later in his remarks that he considered the health care plan a political asset when he ran for president four years ago, but said he realizes now that it is a liability.

Romney spent half of his speech defending the Massachusetts plan -- which required the six percent of the state's citizens who were not insured to obtain health insurance or pay a "bond" of roughly $125 a month -- before turning to a critique of President Obama's health care overhaul passed last year by Congress.

He admitted that some of the aspects of the state plan, which has already led to government price controls to try to rein in costs, have not worked like he had hoped, but concluded: "Overall am I proud of the fact that we did our best for our people and got people insured? Absolutely."

Romney appealed to the conservative belief in Federalism -- based on the 10th amendment to the Constitution -- to defend his actions in Massachusetts. The amendment says that state governments should handle all powers not enumerated to the federal government in the Constitution.

"Our plan was a state solution to a state problem, and [Obama's health law] is a power grab by the federal government to put in a one-size-fits-all plan across the nation," Romney said. He argued that the "bond" requirement in Massachusetts was "a state decision ... to insist on personal responsibility."

But that argument has not been accepted by many conservatives, and the instant feedback to Romney's speech was less than kind from conservatives on Twitter.

The National Review's Jonah Goldberg wrote that Romney was auditioning for the job of David Axelrod, a former top White House adviser to Obama who remains an outside political consultant to the president.

The Wall Street Journal took a big swing at Romney in its pages Thursday, saying that because of his law Massachusetts is spending 40 percent of its budget on health care, versus 30 percent when he signed the bill into law, and that hospitals are giving even higher rates of uncompensated health care than they were then.

Robert Moffit, the director of the conservative Heritage Foundation's Center for Health Policy Studies who worked with Romney on his plan several years ago, told The Huffington Post that Romney "tried to solve a real problem."

"But unfortunately things did not work out for him as he I think he expected," Moffit said. "There was genuinely bad faith implementation. What he wanted, the law did not deliver."