Updated

In a case that could lead to a change in how political campaigns are funded, the Supreme Court Tuesday questioned a key part of federal campaign finance law during arguments over a movie critical of former presidential candidate Hillary Clinton.

Justice Anthony Kennedy, who is often the key vote in any majority decision, suggested the court may be in position to strike down a key part of that law.

"If we think that the application of this....is unconstitutional, then the whole statute should fall," Kennedy said.

This is the third significant case before the Supreme Court to challenge the 2002 McCain-Feingold law, which restricts the flow of money into political races.

A government lawyer defending the law also suggested that books could be prohibited in the same way McCain-Feingold restricts the airing of certain political advertisements within 30 days of a primary or 60 days of a general election. That proposition did not sit well with the court's conservative members.

"That's pretty incredible," Justice Samuel Alito exclaimed.

Justice Antonin Scalia who has consistently ruled against restrictions on campaign speech rhetorically asked the government lawyer if the First Amendment "cover[s] the right of any individual to -- to write, to publish?

"The only question is who's paying for it," Justice Stephen Breyer said as he tried to focus the court's attention on the law's prohibition of union or corporate money used for campaign expenditures.

Instead, political action committees may run ads provided they are not within the proscribed period. "What you do is put limitations on the payment for them. See that there are other ways of paying through it, say as PACs, and then limit very carefully the media that are affected and the times for which they are affected," Breyer said.

Citizens United sought to distribute the film through a cable provider's on-demand system. But in enforcing McCain-Feingold, the Federal Election Commission prohibited the non-profit corporation from completing its cable deal because the film was considered "prohibited electioneering communication."

Scalia took further exception to this ban because an on-demand movie requires the viewer to press a button to watch the movie--unlike a campaign commercial that randomly appears on one's television screen.

"The First Amendment interest is greater when what the government is trying to stifle is not just a speaker who wants to say something but also a hearer who wants to hear what the speaker has to say," Scalia said.

The FEC's conclusion that the movie was nothing more than an overt attempt to persuade voters not to side with Hillary Clinton was affirmed by a three judge panel last summer which ruled the film had "no other interpretation" other than as an advocacy message to voters that Clinton should not be elected president.

Several justices reached the same conclusion on Tuesday.

"This sounds like campaign advocacy," Justice David Souter. "If that isn't an appeal to voters, I can't imagine what is," Justice Ruth Bader Ginsburg added.

But Citizens United lawyer Ted Olson told the court that ruling and the law itself smothered his client's First Amendment rights to free speech. He added the government must have a "heavy burden" in limiting the speech.

"The government cannot prove and has not attempted to prove that a 90-minute documentary made available to people who choose affirmatively to receive it..." Olson said. "Indeed, this documentary is the very definition of robust, uninhibited debate about a subject of intense political interest that the First Amendment is there to guarantee."

Earlier this decade, the court ruled that the McCain-Feingold law's prohibition on corporate electioneering communications, as well as its reporting, disclosure, and disclaimer requirements were not facially invalid. The court later clarified its position to say that only messages of "express advocacy" or its functional equivalent could be prohibited.

A decision is expected by June.