Brad Smith in the Atlantic:

Notable, however, was that the Supreme Court’s progressive wing dissented not merely from the judgment, but from the grant of certiorari itself, because there was no “significant possibility of reconsideration” of Citizens United by the majority. The Court’s progressive wing, lacking the votes to overturn the Citizens United precedent, would not even concede that the decision, until overruled, should be enforced.

So Citizens United appears to be a goner at the first opportunity. But how much difference will that make? Much less than people think.