Here are his thoughts via The New York Times:

He said the court had made a disastrous wrong turn in its recent string of campaign finance rulings.“The voter is less important than the man who provides money to the candidate,” he said. “It’s really wrong.”

He talked about what he called a telling flaw in the opening sentence of last month’s big campaign finance ruling. He filled in some new details about the behind-the-scenes maneuvering that led to the Citizens United decision. And he called for a constitutional amendment to address what he said was the grave threat to American democracy caused by the torrent of money in politics.

And:

Chief Justice John G. Roberts Jr. started his controlling opinion with a characteristically crisp and stirring opening sentence: “There is no right more basic in our democracy than the right to participate in electing our political leaders.”But that was misleading, Justice Stevens said. “The first sentence here,” he said, “is not really about what the case is about.”

And his solution:

“Essentially,” he wrote, “five justices were unhappy with the limited nature of the case before us, so they changed the case to give themselves an opportunity to change the law.”
Continue reading the main storyThe occasion for our talk was Justice Stevens’s new book, “Six Amendments: How and Why We Should Change the Constitution.” One of those amendments would address Citizens United, which he wrote was “a giant step in the wrong direction.”

The new amendment would override the First Amendment and allow Congress and the states to impose “reasonable limits on the amount of money that candidates for public office, or their supporters, may spend in election campaigns.”

Essentially, in so many words, Stevens has said that the conservative majority’s decision were half-baked, twisted and simply a way to hand over the keys of whatever democracy is left to the very powerful and wealthy.No surprise to most of us–but noteworthy coming from someone who was breathing the same air.