Washington (CNN) A Supreme Court move on Tuesday could mean that so-called "dark money" groups would have to disclose the names of their donors, potentially chipping away at a longstanding shroud of secrecy surrounding money in politics.

The move also creates uncertainty around those groups' efforts to raise and spend money just seven weeks from the midterm elections.

The court let stand a federal judge's decision to throw out a decades-old Federal Election Commission regulation that only required nonprofit groups to name their donors when their contributions were earmarked for specific advertisements.

The ruling was a blow to Crossroads GPS, the conservative nonprofit linked to Republican strategist Karl Rove that was involved in the case and had sought the emergency stay. It means nonprofit groups like Crossroads GPS could face new requirements to reveal their donors before November's midterm elections. Crossroads GPS is the No. 2 "dark money" spender since the 2010 Citizens United vs. FEC ruling, according to a recent report by the nonprofit campaign finance group Issue One.

FEC Chairwoman Caroline Hunter told The Washington Post that the names of donors who give money to nonprofits to be used in political campaigns beginning Wednesday will have to be reported. Hunter did not respond to CNN's request for comment.

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