 -- Originally, Chief Justice John Roberts had decided to strike down the health care reform law's individual mandate after oral arguments in March, CBS News reported over the weekend. But sometime over the next six weeks, he changed his mind, bitterly disappointing the his fellow conservatives on the court.

CBS News' Jan Crawford reported that two unnamed sources with knowledge of the deliberations told her that Justice Anthony Kennedy, usually the court's swing vote, launched a "relentless" but unsuccessful campaign to get Roberts to come back to the fold after he changed his mind and joined the court's liberal wing. Kennedy was flatly opposed to the law, and joined the three conservative justices in writing an unsigned dissent that pointedly refused to engage with Roberts' majority opinion. The justices did not sign on to any part of Roberts' opinion, even portions with which they agreed. (Earlier reports that their dissent was originally the majority opinion were wrong, Crawford writes.)

Crawford did not find out why Roberts switched his vote, but she floated two theories.

The first is that Roberts, unlike some other justices, pays attention to media coverage and is highly "sensitive" to the court's public image and legacy. If Roberts had struck down the law, many court-watchers and legal scholars would have derided the court as activist and over-partisan, a characterization some were already making following the Citizens United decision, which swept away decades of Congressionally-passed campaign finance as a violation of the First Amendment.

The second is that Roberts could not have relied on precedent in striking down the law, because Congress had never passed a law quite like health care reform. He would have to write his own theory in overturning the law, which would have opened him up to criticism of judicial overreach.

Longtime courtwatcher and Yahoo News favorite Lyle Denniston agrees with the first theory, writing that he believes Roberts was "determined to try to uphold some key parts of the law...partly because (maybe mainly because) he has grown concerned about the public perception that his Court is a partisan-driven Court." The Wall Street Journal editorialized that if Roberts did bow to outside pressure when he switched his vote it would be a stain on the court's legacy.

But Randy Barnett, a Georgetown professor and the architect of the legal challenge to health care reform, calls the decision a "stunning victory" for his cause. Roberts sided with the conservative wing in ruling that the Commerce Clause did not empower Congress to compel Americans to buy health insurance, instead upholding the mandate as a tax. The Commerce Clause was so widely believed to bestow this power before the oral arguments that Barnett was actually teased by two Harvard constitutional law professors in a forum about the law last year. Barnett's view has quickly gone from fringe to the law of the land, and it may mean that Congress will have to rely on legislating new taxes, which are politically unpopular, in the future if they want to pass similar regulations. With his decision, Roberts was able to significantly limit Congress' power under the Commerce Clause while simultaneously avoiding the appearance of activism.

In a brief appearance on Friday, Roberts did his best to fend off speculation. He said he was heading for a two-week trip to Malta to teach classes. "Malta, as you know, is an impregnable island fortress. It seemed like a good idea," he joked.