Some revealing reporting from Jan Crawford.

Chief Justice John Roberts initially sided with the Supreme Court’s four conservative justices to strike down the heart of President Obama’s health care reform law, the Affordable Care Act, but later changed his position and formed an alliance with liberals to uphold the bulk of the law, according to two sources with specific knowledge of the deliberations.

Roberts then withstood a month-long, desperate campaign to bring him back to his original position, the sources said. Ironically, Justice Anthony Kennedy – believed by many conservatives to be the justice most likely to defect and vote for the law – led the effort to try to bring Roberts back to the fold.

“He was relentless,” one source said of Kennedy’s efforts. “He was very engaged in this.”

But this time, Roberts held firm. And so the conservatives handed him their own message which, as one justice put it, essentially translated into, “You’re on your own.”

The conservatives refused to join any aspect of his opinion, including sections with which they agreed, such as his analysis imposing limits on Congress’ power under the Commerce Clause, the sources said.

Instead, the four joined forces and crafted a highly unusual, unsigned joint dissent. They deliberately ignored Roberts’ decision, the sources said, as if they were no longer even willing to engage with him in debate.

The inner-workings of the Supreme Court are almost impossible to penetrate. The Court’s private conferences, when the justices discuss cases and cast their initial votes, include only the nine members – no law clerks or secretaries are permitted. The justices are notoriously close-lipped, and their law clerks must agree to keep matters completely confidential.

But in this closely-watched case, word of Roberts’ unusual shift has spread widely within the Court, and is known among law clerks, chambers’ aides and secretaries. It also has stirred the ire of the conservative justices, who believed Roberts was standing with them.

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Because Roberts was the most senior justice in the majority to strike down the mandate, he got to choose which justice would write the Court’s historic decision. He kept it for himself.

Over the next six weeks, as Roberts began to craft the decision striking down the mandate, the external pressure began to grow. Roberts almost certainly was aware of it.

Some of the conservatives, such as Justice Clarence Thomas, deliberately avoid news articles on the Court when issues are pending (and avoid some publications altogether, such as The New York Times). They’ve explained that they don’t want to be influenced by outside opinion or feel pressure from outlets that are perceived as liberal.

But Roberts pays attention to media coverage. As Chief Justice, he is keenly aware of his leadership role on the Court, and he also is sensitive to how the Court is perceived by the public.

There were countless news articles in May warning of damage to the Court – and to Roberts’ reputation – if the Court were to strike down the mandate. Leading politicians, including the President himself, had expressed confidence the mandate would be upheld.

Some even suggested that if Roberts struck down the mandate, it would prove he had been deceitful during his confirmation hearings, when he explained a philosophy of judicial restraint.

It was around this time that it also became clear to the conservative justices that Roberts was, as one put it, “wobbly,” the sources said.

It is not known why Roberts changed his view on the mandate and decided to uphold the law. At least one conservative justice tried to get him to explain it, but was unsatisfied with the response, according to a source with knowledge of the conversation.

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Regardless of his thinking, it was clear to the conservatives that Roberts wanted the Court out of the red-hot dispute.

Roberts had begun to focus on a different argument to uphold the law and the mandate’s penalty by defining it as a tax. That strained argument had received almost no attention in the lower courts, which had uniformly rejected it. It was seen as a long-shot by the law’s supporters.

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Roberts then engaged in his own lobbying effort – trying to persuade at least Justice Kennedy to join his decision so the Court would appear more united in the case. There was a fair amount of give-and-take with Kennedy and other justices, the sources said. One justice, a source said, described it as “arm-twisting.”

Even in Roberts’ opinion, which was circulated among the justices in early June, there are phrases that appear tailored to get Kennedy’s vote. Roberts even used some of the same language that Kennedy used during oral arguments.

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In fact, Kennedy was the most forceful and engaged of all the conservatives in trying to persuade Roberts to stand firm to strike down the mandate. Two sources confirm that he didn’t give up until the very end.

But Roberts didn’t focus entirely on Kennedy, the sources said. He tried to persuade the conservatives to join at least the parts of his opinion with which they agreed, such as his Commerce Clause analysis.

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But despite Roberts’ strong language on the Commerce Clause, the conservatives would have none of it, the two sources said, even though there was no significant difference in their reasoning on that issue.

Indeed, since the four conservatives agreed the mandate went beyond the commerce power, the Court now has five Justices who would constrain what Congress can do going forward – imposing significant limits on federal power.

The majority decisions were due on June 1, and the dissenters set about writing a response, due on June 15. The sources say they divided up parts of the opinion, with Kennedy and Scalia doing the bulk of the writing.

The two sources say suggestions that parts of the dissent were originally Roberts’ actual majority decision for the Court are inaccurate, and that the dissent was a true joint effort.

The fact that the joint dissent doesn’t mention Roberts’ majority was not a sign of sloppiness, the sources said, but instead was a signal the conservatives no longer wished to engage in debate with him.