Constitutionality of PPACA

It challenges the federal government's vast expansion of Medicaid as ''an unprecedented encroachment on the sovereignty of states.''

The Justice Department plans to counter that states do not have to participate in Medicaid, according to sources familiar with its thinking.

But the states argue that their health care systems have grown so dependent on Medicaid that withdrawing would be catastrophic.

A second count attacks the tax penalty on the uninsured, saying it is an illegal direct tax, and not an allowable excise tax on goods or services.

But the central challenge concerns the Supreme Court's interpretation of the Commerce Clause, as expressed in four decisions handed down over 63 years. If the court interprets the clause broadly, as it did in two seminal cases on the subject, the health insurance mandate is likely to survive.

In those two cases, Wickard v. Filburn in 1942 and Gonzales v. Raich in 2005, the court ruled that Congress's regulatory authority was so extensive that it could even prevent growers from cultivating crops for personal use because of the cumulative impact on the market.