Bachmann and other Republicans have long said the so-called individual mandate in the national health care overhaul is unconstitutional. The Minnesota Republican and presidential candidate extended that criticism to similar requirements at the state level.
"I believe it is also unconstitutional for states to mandate as a ... condition of citizenship, that an individual would have to purchase a product or service even at the state government's behest," she said at a GOP presidential candidates' forum in South Carolina.
The criticism is a shot at Romney, who, as governor of Massachusetts, helped pass a health care law that included the requirement to buy health insurance. President Obama's aides, causing a bit of mischief in the GOP race, have cited the law as an inspiration in the past.
After being pressed, Bachmann was not able to point to a particular section of the U.S. Constitution that she thinks an individual mandate at the state level would violate. "I believe that it's inherent in the Constitution," she said.
Critics of the national mandate typically allege it violates the Commerce Clause giving Congress the power to regulate commerce -- opponents say those who don't purchase health insurance are not participating in commerce and therefore should not be regulated. However, the clause does not mention state governments.
Romney, the last candidate to take questions, defended his record Monday. He said the key difference between his state's approach and Obama's was his was limited in scope.
"The critical thing is this ... We dealt with 8 percent (of state residents). He dealt with 100 percent of American people," Romney said. "It's simply unconstitutional, it's bad law, it's bad medicine."
Romney said that if elected, he'd order his health secretary on "day one" to grant a waiver to the states to opt out of the federal health care law.
Romney, meanwhile, kept his criticism trained on Obama. Asked at one point what he'd changed about U.S. foreign policy, he said: "A lot -- first, I'd have one." Romney accused Obama of exercising a "reactive" foreign policy during the Arab Spring and throwing Israel "under the bus."
The forum Monday was organized in part by Tea Party-aligned Sen. Jim DeMint, R-S.C. DeMint, Rep. Steve King, R-Iowa, and American Principles Project founder Robert George questioned five of the candidates on topics ranging from abortion to the deficit to immigration.
Texas Gov. Rick Perry, who entered the race last month and has surged to the front of the field in most national polls, was slated to attend but canceled just hours before the event in order to deal with wildfires in Texas.
At the forum, businessman Herman Cain pitched himself as the candidate who can get the private sector humming. He called for reducing corporate income taxes and personal income taxes, while instituting a national sales tax. He said such a system would help prevent the government from picking winners and losers in the tax code. Cain also backed DeMint's call for a balanced-budget amendment to the Constitution.
Former House Speaker Newt Gingrich also issued a stern warning on the economy, suggesting new policies will be needed soon to prevent a tailspin.
"This economy is in grave danger of getting worse, not getting better. Nobody should assume that 9 percent (unemployment) is the bottom," he said.
True to form, Texas Rep. Ron Paul sharply criticized the policies of the Federal Reserve Monday and called for a much smaller military footprint overseas.
Paul, who elicited a supportive shout from the crowd when he called for bringing the troops home, suggested that doing so could help the economy.
"Let the troops come home and spend their money here," Paul said. "Now that would be a monetary injection I could support."