The case, which arises from a political documentary called "Hillary: The Movie," could potentially transform the way political campaigns are conducted.

At issue is whether the court should overrule its decision in Austin vs. Michigan Chamber of Commerce, which upheld restrictions on corporate spending to support or oppose political candidates.

The court's June order calling for the re-argument, which will take place Sept. 9, has generated more than 40 friend-of-the-court briefs that debate whether corporations should be free to spend millions of dollars to support candidates.

"This is rough business," Fred Wertheimer, a veteran advocate of tighter campaign regulations, told the New York Times. "We're not dealing with campaign finance laws. We're dealing with the essence of power in America."

The case involves "Hillary: The Movie," a documentary by the conservative Citizens United that paints a negative portrait of Sen. Hillary Rodham Clinton. The group lost a lawsuit with the U.S. Federal Election Commission when it sought permission to distribute the film on a video-on-demand service. A lower court agreed with the FEC's position.

During Supreme Court arguments last spring, one government lawyer argued that if the government could prevent the film's distribution, it could also hypothetically make it a crime to distribute books advocating the election or defeat of political candidates -- so long as they were paid for by corporations and not their political action committees.

"That position seemed to astound several of the more conservative justices, and there were gasps in the courtroom," the Times reported.

The discussion of book banning may have helped prompt the request for re-argument.