With multiple lawsuits challenging the constitutionality of ObamaCare's "individual mandate," the law's backers have proffered two principal arguments in its defense.  First, they claim that Congress has the power to require U.S. residents to purchase health insurance under the Constitution's grant of power "to regulate Commerce...among the several States."  Second, they claim the measure is authorized by the taxing power.

Regarding the commerce power, Cato senior fellow Randy Barnett explains in yesterday's Wall Street Journal:

[T]he Court has never upheld a requirement that individuals who are doing nothing must engage in economic activity by entering into a contractual relationship with a private company. Such a claim of power is literally unprecedented.

Barnett also explains that the text of the law precludes ObamaCare's defenders from claiming that the individual mandate is authorized by the taxing power.  The individual mandate defines a minimum level of coverage and then imposes a penalty on people who do not purchase such coverage.  Barnett notes that the law invokes the commerce power (not the taxing power) to justify the mandate, and refers to the penalty for non-compliance as a "penalty" (not a tax):

In short, the "penalty" is explicitly justified as a penalty to enforce a regulation of economic activity and not as a tax. There is no authority for the Court to recharacterize a regulation as a tax when doing so is contrary to the express and actual regulatory purpose of Congress.

At National Review Online, Cato chairman Bob Levy explains, "even if the penalty for noncompliance is deemed to be a tax rather than a fine, it does not meet the constitutional requirements for income, excise, or direct taxes," and would be an unconstitutional tax.

That leaves ObamaCare's supporters to defend the individual mandate as an (unprecedented) use of the commerce power.  Barnett writes:

Are there now five justices willing to expand the commerce and tax powers of Congress where they have never gone before? Will the Court empower Congress to mandate any activity on the theory that a "decision" not to act somehow affects interstate commerce? Will the Court accept that Congress has the power to mandate any activity so long as it is included in the Internal Revenue Code and the IRS does the enforcing?

Yes, the smart money is always on the Court upholding an act of Congress. But given the hand Congress is now holding, I would not bet the farm.

Levy concludes, "Legal refinements aside, the insurance mandate is an affront to personal liberty that will exacerbate our health-care problems. For those who care, it’s unconstitutional as well."