The case involves “Hillary: The Movie,” a mix of advocacy journalism and political commentary that is a relentlessly negative look at Mrs. Clinton’s character and career. The documentary was made by a conservative advocacy group called Citizens United, which lost a lawsuit against the Federal Election Commission seeking permission to distribute it on a video-on-demand service. The film is available on the Internet and on DVD. The issue was that the McCain-Feingold law bans corporate money being used for electioneering.
The right position for the Court is that McCain-Feingold, and all other campgain finance regulation, constitutes unconstitutional limitation on free speech. This means reversing the Court's 1976 Buckley v. Valeo decision, which held that government limits on campaign spending were unconstitutional but limits on contributions were not.
This distinction is meaningless. If it is OK for a millionaire to spend his own money promoting his own campaign, why can he not give that money to somone else, who might be a more effective adovocate for that millionare's views, so that this other person can run for office?
More broadly, campaign finance regulation is thought control: it takes a position on whether money should influence political outcomes. Whether or not one agrees, this is only one possible view, and freedom of speech is meant to prevent government from promoting or discouraging particular points of view.
It would be a brave step for Court to reverse Buckley, but it is the right thing to do.
Addendum: A nice youtube video from Cato on this issue.