Is Social Security privatization unconstitutional?

From Jonathan Cohn comes one of those things that you read and wonder, "Why didn't I think of that?"

The idea was to take Social Security, a mandatory public pension program, and turn it into a system of mandatory personal investment accounts...

Conservatives presumably thought privatization was constitutional; otherwise, they would not have worked so feverishly to enact it. But if the principle holds for old-age insurance, it ought to hold for medical insurance, too. In other words, if it’s ok for the government to make you pay for regulated private investments, then it should be ok for the government to make you pay for regulated private health insurance. Yet, as far as I can tell, the folks who spent all of those years promoting Social Security as an all-American, free market innovation are the same ones that now insist the Affordable Care Act is an unprecedented threat to liberty.