Just as Sen. John McCain does a slow fade from the public spotlight, so, too, does his signature law, the McCain-Feingold campaign restrictions, slowly but steadily lose both its power and its luster.

The latest potential dimming of McCain-Feingold’s bright lines against “outside” campaign speech came Friday courtesy of the Supreme Court’s agreement to hear a case called Citizens United v. Federal Election Commission.

McCain-Feingold, of course, is famous for prohibiting corporate (“soft money”) campaign donations and for limiting the ability of outside groups to run campaign ads within 60 days of an election (or 30 days of a primary) without submitting themselves to manifold Federal Election Commission regulations.

Citizens United is a conservative advocacy organization that has adopted a new approach to getting its message out. Rather than use the typical 30-second campaign ad or issue ad, CU has begun making full-length documentaries, several per year, on a wide variety of issues ranging from the United Nations to the original interplay between faith and government to a defense of the war against terrorists.

CU ran into McCain-Feingold last winter when it produced “Hillary: The Movie,” a harsh look at the former first lady. (Full disclosure: I briefly worked for CU and did the final editing for a book released in connection with the movie.) CU, a non-profit organization, wanted to run television brief commercials to promote its movie. After all, that’s how movies are marketed, through advertising.

The problem was that CU was marketing the movie during the primary season. Because Hillary was a candidate in those primaries, the Federal Election Commission (FEC) told CU it couldn’t air its commercials without abiding by the commission’s burdensome regulations. (It’s obviously hard to advertise a movie called “Hillary” without mentioning, yes, Hillary.)

A federal district court three-judge panel agreed, ruling that, since the movie clearly could have an effect on Sen. Clinton’s election prospects, an ad for the movie amounted to electioneering of the sort McCain-Feingold made illegal.

The movie therefore would be prohibited (unless CU abided by the burdensome reporting requirements and disclaimers required by the FEC). Furthermore, the panel ruled that the whole movie itself could not be broadcast, for the same reasons.

CU appealed to the Supreme Court. James Bopp, Jr., CU’s lawyer, had served as lead counsel in a 2007 case, Wisconsin Right to Life v. FEC, in which the Supreme Court had effectively blown a hole through the McCain-Feingold campaign speech restrictions as interpreted by the FEC.

In the Wisconsin case, the court noted that the only speech subject to regulations is “express advocacy,” and ruled that “an ad is the functional equivalent of express advocacy only if the ad is susceptible of no reasonable interpretation other than as an appeal to vote for or against a specific candidate.”

Bopp said yesterday in an interview with this columnist that the Wisconsin case effectively limited McCain-Feingold’s speech restrictions to only about 20 percent of the breadth formerly asserted by the FEC.

In his brief to the Supreme Court, Bopp quoted from Wisconsin case: “The benefit of any doubt,” the Supreme Court wrote, “[goes] to protecting rather than stifling speech.”

Put more bluntly, the court wrote back in the Buckley v. Valeo case of 1976, “the tie goes to the speaker, not the censor.”

In the end, that’s what is at issue here: Government censorship of politically tinged speech. Yet that is exactly the sort of speech the First Amendment was expressly designed to protect.

What’s worse in the CU case is that not only would the FEC deter CU from getting out its message, but also it would keep it from even advertising the fact that it has a message to get out.

A documentary is a serious enterprise. It costs lots of money to produce and distribute. To deny the rights of documentarians to recoup their investment by selling their product – and thus to deny them the right to a lengthy, as opposed to soundbite-level, examination of public issues – is to snuff out free speech as effectively as any Internet ban in Red China.

The nation’s high court will hear this case in February. By rights it ought to rule in CU’s favor. The prohibition on CU’s ads is a pernicious threat to open public debate.

Quin Hillyer is associate editorial page editor of The Washington Examiner.