The Medicaid expansion is also an easy case under existing law. It offers the states new federal funds in return for complying with new federal eligibility requirements that further Medicaid's basic goal of providing health care coverage to the needy. Many federal programs -- including parts of Social Security and federal grants to educational institutions -- have been expanded over the years in similar ways. It is hard to write an opinion that jettisons Medicaid expansion and does not threaten them as well.

If the justices uphold the Affordable Care Act, they will be doing pretty much what they normally do: defending the constitutional assumptions of the current political regime. Moreover, they will be upholding a recent act of Congress that is also the current president's signature legislative achievement. And they will be reaffirming many decades of settled precedent. All in all, there are multiple and overlapping forces pulling toward this result.

But are there situations where the Supreme Court has acted contrary to these basic tendencies? Are there cases where, in an issue of national prominence, the Court has struck down a recent and highly visible act of Congress when the Court was not defending the existing constitutional regime and its basic commitments?

When we look at the history of the Supreme Court, it's hard to find many examples. Bush v. Gore doesn't count. It didn't strike down a congressional statute at all. It just stopped recounts ordered by the Florida courts, effectively handing George W. Bush the presidency, for good or for ill. The 1857 decision in Dred Scott v. Sandford is universally reviled today, but when the Court struck down the Missouri Compromise of 1820 it was doing the bidding of a national political coalition dominated by defenders of slavery, who believed that the compromise had outlived its usefulness. (In fact, Congress had already repealed the Missouri Compromise in 1850, although the facts of Dred Scott occurred while it was still in effect.)

When the justices struck down Franklin Roosevelt's National Recovery Act and other New Deal legislation in the mid 1930s, they were actually defending the constitutional assumptions of the existing regime, which, since 1896, had mostly been dominated by conservative Republicans. It was Roosevelt and his New Dealers who were the insurgents. Only after Roosevelt won increasing Congressional majorities in three successive elections and demonstrated that the public was behind his reforms did the Supreme Court make its famous "switch in time" in 1937, upholding federal labor laws and state minimum wage laws.

Then, after Roosevelt was finally able to make new appointments, the Supreme Court -- by now stocked with New Dealers -- upheld all of his signature legislative achievements. In fact, Roosevelt's actions -- and his nine Supreme Court appointments -- began a new constitutional regime, with new assumptions about federal power and judicial review.