Can someone please tell me the constitutional basis for imposing a penalty or tax on someone for not buying health care? I've asked my lawyer friends and they don't know. I called my congressman's office and they don't know. Reporters have raised the issue with the White House and they haven't addressed it with their lawyers (which, if true strikes me as incompetent, especially since our President was apparently a constitutional law professor).

Some member of Congress (a conservative) introduced a bill to require that all proposed legislation must have as a preamble, the Constitutional basis for the proposals in the bill. Of course it was sweapt aside and never saw daylight. Why let a crappy little thing like the Constitutional basis for legislation slow down the grinding out of one bill after another! Congress has important work to do...

It can be effectively argued that Congress has been granted the authority to require that individuals purchase health insurance under the Commerce Clause of the Constitution. (Article 1 Section 8). I’ll spare you the google search:

Section 8. The Congress shall have power to lay and collect taxes, duties, imposts and excises, to pay the debts and provide for the common defense and general welfare of the United States; but all duties, imposts and excises shall be uniform throughout the United States…

Levying a tax penalty against citizens who fail to enroll in a program which provides for the “general welfare” falls under the purview of this clause. Of course, this authority has been challenged and has so far been upheld by SCOTUS over the last several decades. Take a look at Hospital Bldg. Co v. Rex Hospital Trustees (1976). But perhaps a little more directly, the establishment of Social Security was challenged and upheld by SCOTUS in Helvering v. Davis (1937) which arguably also gives Congress the power to establish a national social insurance program.

For some scholarly research on the matter, Georgetown has a link to some interesting papers on the subject.

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While President Obama is likely aware of this Article since, as you point out, he was a Constitutional Law Professor, this clause of the Constitution addresses the authority of Congress, not the President. So his legal team wouldn’t necessarily be called on to address it.