Up until last Thursday, if you had told any Supreme Court observer that Justice Anthony Kennedy would side with the conservatives and find Obamacare's individual mandate unconstitutional, he or she would probably have assumed that the law would be struck down. But that's not how the court ended up ruling.

Instead, the supposedly conservative Chief Justice John Roberts sided with the court's four liberal members and upheld Obamacare's individual mandate as a proper use of Congress' taxing power. Never mind that both Congress and the president had forcefully and repeatedly denied that the individual mandate was a tax.

So what happened?

George Mason University Law School professor David Bernstein, and others, have collected a sizable amount of evidence suggesting Roberts did originally side with his conservative brethren on the law, but flip-flopped late in the process.

First, Bernstein notes, the conservative dissent has a long and well developed section on why the court should strike down the whole law and not just the mandate. If the dissenters knew they were writing a dissent, this section would likely never have been written. Since the mandate was upheld, it is completely irrelevant.

Second, Justice Antonin Scalia's response to Roberts' decision upholding the mandate under the taxing power is tacked on at the end of the dissent. If this was Roberts' position from the beginning, why did Scalia wait so long and spend so little time rebutting it?

Finally, the conservative dissent repeatedly refers to Justice Ruth Bader Ginsburg's opinion as a "dissent." But Ginsburg was on the winning side. Unless Roberts changed his mind late in the game, it should have been called a concurrence.

So why did Roberts, who was appointed by President George W. Bush, betray his movement when it was on the verge of a truly historic and consequential victory?

A clue can be found in the Washington Post, where Ezra Klein lauds Roberts as "a political genius" for upholding President Obama's signature domestic accomplishment:

"By voting with the liberals to uphold the Affordable Care Act, Roberts has put himself above partisan reproach. No one can accuse Roberts of ruling as a movement conservative. He's made himself bulletproof against insinuations that he's animated by party allegiances."

In other words, Roberts sold his vote in order to increase his own power and stature. But for what purpose? Klein suggests he will use his new influence to further the conservative goal of "imposing limits on federal power."

And there is some evidence that has already happened. Roberts was able to get two of the liberal justices, Elena Kagan and Stephen Breyer, to sign on to a much-overlooked part of his decision allowing states to opt out of Obamacare's new Medicaid requirements.

Since the 1987 South Dakota v. Dole decision finding that Congress could force states to adopt a national drinking age in exchange for highway funds, the federal government has been free to dictate state policies in many areas by threatening to withhold vital federal funding.

Roberts' Obamacare/Medicaid decision, a 7-2 ruling, puts some real limits on that power for the very first time. By itself, that would have been a huge victory for conservatives.

But the Medicaid portion of Roberts' decision would never have been written if he would have just sided with the other four conservative justices on the mandate to begin with. All four were ready to throw the whole law out.

Maybe that would have diminished the prestige of the court. Maybe Roberts has made an astute political calculation that will ultimately serve a higher cause in the future.

But it is a huge gamble, and a transparently political one at that. Roberts' slippery maneuvering may end up doing more damage to the court's reputation than if he had just stuck with his original conservative vote.

Conn Carroll (ccarroll@washingtonexaminer.com) is a senior editorial writer for The Washington Examiner. Follow him on Twitter at @conncarroll.