Nonprofit political groups engaged in electioneering activities must disclose their top political donors beginning this week after the Supreme Court on Sept. 18 declined to intervene in a controversial campaign finance case.

The court’s decision could affect political ad spending by 501(c)(4) and 501(c)(6) groups from now until the November midterm elections.

“This is a real victory for transparency,” said Ellen Weintraub, vice chairwoman of the Federal Election Commission. “As a result, the American people will be better informed about who’s paying for the ads they’re seeing this election season.”

The ruling lets stand a lower court ruling forcing politically active nonprofit groups to disclose the identity of any donor giving more than $200 to influence elections. The original complaint was filed by Citizens for Responsibility and Ethics in Washington (CREW) against Crossroads GPS, a conservative 501(c)(4) group that seeks to boost Republican political candidates. CREW alleged that Crossroads was violating federal law by keeping its donors secret. Last month, U.S. District Court Judge Beryl Howell ruled in CREW’s favor and gave the FEC 45 days to draft a new regulation, which it has not done because the case remains under appeal.

Supporters of deregulating campaign finance said the Supreme Court’s decision not to intervene means that voters will have a lot less information on candidates running for office this fall.

“It’s unfortunate that citizens and groups who wish to advocate for their candidate will now have to deal with a lot of uncertainty less than two months before the election,” said FEC Chairwoman Caroline Hunter in an interview with the Washington Post.