The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
National health care obviously falls into the "general Welfare of the United States". It would be amusing to see somebody try to sue on that point though, because the government would probably have to admit that a mandate to buy insurance was a form of tax in court.
Jay