WASHINGTON - The US Supreme Court on Monday declined to publish an opinion on a documentary titled "Hillary: The Movie," in a freedom of expression case on whether films critical of political candidates can be regulated during campaign season.

The justices ordered both sides to file new arguments in September that address whether two legal precedents on election law should be overruled.

The case pits the Federal Election Commission (FEC) against Citizens United, a conservative group that made a biting film last year about then White House contender Hillary Clinton, but was barred by the FEC from airing ads for it during the presidential election.

The FEC ruled the producers had to conform to existing US laws on campaign financing, which require political adverts to reveal the source of their funding.

Federal judges ruled in favor of the FEC, but in a Supreme Court hearing in March the justices appeared to be leaning in favor of loosening federal controls on spending by corporations and labor unions during elections.

The new hearing was set for September 9, a month before the Supreme Court starts its new term.