It's the judiciary's Nixon in China: Chief Justice John Roberts joins the liberal wing of the Supreme Court and upholds Obamacare.

How? By pulling off one of the great constitutional finesses of all time. He managed to uphold the central conservative argument against Obamacare while at the same time finding a narrow definitional dodge to uphold the law — and thus prevented the court from being seen as having overturned, presumably on political grounds, the signature legislation of this administration.

Why did Roberts do it? Because he carries two identities. Jurisprudentially, he is a constitutional conservative. Institutionally, he is chief justice and sees himself as uniquely entrusted with the custodianship of the court's legitimacy, reputation, and stature.

As a conservative, he is as appalled as his conservative colleagues by the administration's central argument that the individual coverage mandate is a proper exercise of the authority to regulate commerce.

That makes congressional power effectively unlimited. Mr. Jones is not a purchaser of health insurance. Mr. Jones has therefore manifestly not entered into any commerce. Yet Congress tells him he must buy health insurance — on the grounds that it is regulating commerce. If government can do that under the Commerce Clause, what can it not do?

"The framers ... gave Congress the power to regulate commerce, not to compel it," Roberts wrote. Otherwise, you "undermine the principle that the federal government is a government of limited and enumerated powers."

That's Roberts, philosophical conservative. But he lives in uneasy coexistence with Roberts, custodian of the court, acutely aware that the judiciary's arrogation of power has eroded the esteem in which it was once held. Most of this arrogation occurred under the liberal Warren and Burger courts, most egregiously with Roe v. Wade.

More recently, however, few decisions have occasioned more rancor than Bush v. Gore. It was seen by many as a political act disguised as jurisprudence. Whatever one thinks of its substance, it did affect the court's reputation.

Roberts seems determined that there be no recurrence. Hence his straining in his Obamacare ruling to avoid a similar result — a 5-4 decision that might be perceived as partisan and political.

National health care has been a liberal dream for a hundred years. It is the most significant piece of social legislation in decades. Roberts' concern was that the court do everything it could to avoid being seen, rightly or wrongly, as high-handedly overturning sweeping legislation passed by both houses of Congress and signed by the president.

How to reconcile the two imperatives? Find a finesse that upholds the law, but only on the narrowest grounds — interpreting the mandate as a tax, something generally within Congress' power.

Result? The law stands, obviating any charge of partisanship, and yet the Commerce Clause is reined in. Roberts draws the line against a decades-old expansion of congressional power.

That's not how I would have ruled. I think the "mandate is merely a tax" argument is a flimsy dodge. (The "tax" is obviously punitive, regulatory, and intended to compel.) Perhaps that's not how Roberts would have ruled had he been just an associate justice. But that's how he did rule.

Now there's only one way Obamacare can be overturned: the same way it was passed, by a new president and a new Congress. That's undoubtedly what Roberts is saying: Your job, not mine. I won't make it easy for you.