In their essay
Why the Personal Mandate to Buy Health Insurance Is Unprecedented and Unconstitutional, Randy Barnett, Nathaniel Stewart, and Todd Gaziano argue that:
A mandate requiring all individuals to purchase health insurance would be an unprecedented form of federal action. The government has never required people to buy any good or service as a condition of lawful residence in the United States. An individual mandate would have two features that, in combination, would make it unique. First, it would impose a duty on individuals as members of society. Second, it would require people to purchase a specific service that would be heavily regulated by the federal government.
They further argue that:
Nowhere in the Constitution is Congress given the power to mandate that an individual enter into a contract with a private party or purchase a good or service and, as this paper will explain, no decision or present doctrine of the Supreme Court justifies such a claim of power. Therefore, because this claim of power by Congress would literally be without precedent, it could only be upheld if the Supreme Court is willing to create a new constitutional doctrine.
They then go on to explain "why the two powers cited by supporters of this bill--the power of Congress to regulate interstate commerce and the power of Congress to tax--do not justify an individual mandate, even under the most expansive readings given these powers by the Supreme Court."