The states have challenged the constitutionality of the individual mandate, which requires the purchase of insurance, in the ACA. In the summary of its argument, the Justice Department said the individual mandate falls under Congress's right to institute taxes. It also cites the commerce clause in Article I of the Constitution as grounds for upholding the individual mandate.
The Supreme Court is scheduled to hear arguments on the case on March 26-28. A copy of the Justice Department's brief can be found below.