Health-Insurance-Umbrella-Protection 300pxChief Justice John Roberts saved Obama’s Health Care Act today; and the natural question to ask is: Why? Roberts has been a committed movement conservative since his service in the Reagan Administration. His decisions in major cases usually match up with the Republican political agenda. So how to explain his switch in the health care case?

To begin with, it wasn’t a complete switch. Roberts’s opinion upholds the Act’s individual mandate (to buy health insurance or pay a fine to the government) as an exercise of the federal power to tax, but agrees with political critics of the Act and the dissenting conservatives on his Court that the mandate is not a valid exercise of Congress’s power to regulate commerce because it regulates “inactivity” rather than “activity.”

This is really a libertarian argument (the government violates your freedom when it compels you to do things) rather than a commerce-clause argument, since the decisions people make to avoid being insured until they get sick obviously affect the insurance market. Roberts didn’t need to say anything about Congress’s commerce power because he upheld the Act on another ground anyway. I think he wrote his opinion the way he did to toss a symbolic bone to fellow conservatives.

I’d guess that Roberts was trying both (1) to avoid further political damage to the image of his Court and (2) to preserve its capacity to act politically in the longer term. Cases like Bush v. Gore, Citizens United (allowing unlimited corporate spending on political issues), cases upholding restrictions on “partial-birth” abortions and many cases favoring corporations over unions, employees and consumers, have made it seem that Republican ideologues have captured the Court’s majority.

But although most Americans tell pollsters they don’t like “Obamacare,” they do like almost everything that’s in it. The four conservative dissenters in today’s decision would have invalidated the entire Act. Roberts knows perfectly well that a decision overturning Obama’s signature accomplishment in an election year would ignite a forest fire blowing straight toward his Court. The 30 million uninsured people who would have failed to get the benefits promised by the Act in 2014, would all blame the Roberts Court as well. Next year, Roberts is almost certain to lead majorities to strike down the Voting Rights Act, which will help southern states suppress the votes of poor, black, and Latino voters; and to gut what remains of affirmative action in higher education, among other things.

What about the negatives in today’s cases, that (1) Congress may not regulate “inactivity” through the Commerce Clause and that (2) Congress may not “coerce” the states into expanding Medicaid coverage by threatening to withhold funds for existing Medicaid if they refuse to expand? I doubt, though I could well be wrong about this, that these will do much damage. A liberal Congress – if such a thing ever returns – can structure a bill to avoid regulating “inactivity,” especially since today’s decision says it can tax inactivity. And most state governments will probably be unable to resist political pressure to accept what is mostly free money from Washington to expand medical coverage for their poor, elderly and disabled citizens.

Robert W. Gordon is a professor at Stanford Law School.