“The opinion has the merit of being faithful to the notion that money is speech and that out-of-district money has the same First Amendment protection as in-district money,” he said. “I think that’s an incorrect view of the law myself, but I do think there’s a consistency between that opinion and what went before.”

He was referring to the court’s earlier campaign finance decisions and, notably, to Citizens United. How that case was transformed from a minor, quirky case about a tendentious documentary into a judicial landmark has long been a source of mystery.

For starters, the case was argued twice. The first time was in March 2009, meaning it should have been decided by the end of the term that June. I asked Justice Stevens whether he, as the senior justice among the four dissenters, would have assigned the 2009 dissent.

“I would, and I did,” he said. And he more or less confirmed that the assignment went to Justice David H. Souter, as Jeffrey Toobin has reported in The New Yorker. “He was certainly a logical candidate to write the dissent,” Justice Stevens said of Justice Souter, who retired in 2009.

The draft dissent, which has not been made public, questioned the majority’s attempt to recast a modest case into a blockbuster that would overrule major precedents and allow unlimited campaign spending by corporations and unions.

The draft dissent caused the majority to pause, Justice Stevens said, thanks to “the strong expression of the feeling among the dissenters that procedurally the case was not in the proper posture to reach the issue that they ultimately decided.”

“I think it persuaded the majority that it would be better to have a re-argument so that they could not be accused of deciding something that had not been adequately argued,” he said. “And I think they were right to do that.”