John Roberts' decision on health care places the country at a constitutional crossroads. On the one hand, his majority opinion upholds the greatest expansion of the welfare state since the 1960s. On the other, it undermines the constitutional underpinnings of big government. This makes the current presidential election into a constitutional turning point -- the next judicial appointments will determine the path which the nation will follow for the next generation.

Roberts decided the case all by himself. The four liberals found him too conservative on one key issue; the four conservatives found him too liberal on the other. Though nobody agreed with him on both, he cast the decisive fifth vote on each. This is why the next Supreme Court appointments will tip the balance beyond the Chief Justice's control.

Begin with Roberts as a conservative. Since the New Deal, the Court has systematically upheld Congress' power to respond to economic problems that were too big for individual states to handle. For example, before Roosevelt came to power, a conservative Court had struck down Congress' nationwide ban on child labor. So far as the majority was concerned, Congress had abused its power because the children worked in local plants which didn't have a "direct" effect on interstate commerce.

But the New Deal Court repudiated this legalistic talk of "directness." They based Congressional power on the common-sense recognition that the old system encouraged a "race to the bottom" that pressured more and more states to allow the exploitation of children. This sufficed to justify the invocation of the interstate commerce clause.

Roberts' majority opinion, signed by the four conservatives, returns to the logic-chopping of an earlier era. Never mind that a single state which provides universal health care may lose business to other states that allow firms to cut costs by denying health insurance to workers. The Roberts Five doesn't find this a "direct" enough connection to enable the national government to create a level playing field on interstate competition. Similarly, millions of uninsured Americans regularly travel out-of-state and go to emergency rooms at taxpayer cost. But once again, this is not a "direct" enough connection for the five conservatives.

There is no need to scrutinize these, and similar, arguments in detail. The key point: Roberts' opinion will provide a springboard for an even more aggressive assault on national regulation if Mitt Romney is our next president, and appoints conservatives to fill the places of aging liberals like Ruth Ginsburg or Stephen Breyer. At that point, Roberts' savvy caution will no longer deter the Court from a much broader attack on big government.

But Roberts did save the Court this time around. Having blown apart the New Deal underpinnings of modern government, he then saved the Affordable Care Act by upholding the mandate under the federal government's taxing power. Siding with the four liberals this time, he correctly held that Congress' power over taxation is even broader than its power over interstate commerce, and amply supported an annual payment from Americans who wished to remain uninsured.

This means that if Obama wins, his new liberal Court appointments won't find themselves in the embarrassing position of condemning their immediate predecessors. Since Roberts upheld healthcare as a tax, they can simply say that all his legalistic talk about narrowing the commerce clause was unnecessary for decision; and that the new liberal majority will refuse to support any further attempt to lead the country back into the early twentieth century.

Unlike most presidential elections, there are big constitutional stakes riding on this one. There is only one clear winner -- and that is John Roberts himself. If Obama gains a second term, Roberts will retain credibility as a nonpartisan leader. After all, he didn't kill Obamacare when he could have done so easily. If Romney takes the White House, Roberts' opinion on the commerce clause will define the parameters of constitutional development for a long time to come.

Not a bad day's work for a Chief Justice looking to a second decade of leadership on a changing Court. Beyond his remarkable personal victory, Roberts' balancing act has also served the nation. It has preserved the Court's legitimacy as America confronts the crises that are sure to arise in the coming decades.

Bruce Ackerman is professor of law and political science at Yale and the author of We the People.