The court set aside an order issued by Chief Justice John Roberts on Saturday. The social welfare group Crossroads GPS, a defendant in the lawsuit, had fought to stall disclosure while it prepares to appeal. It failed in the D.C. Circuit Court of Appeals and then appealed to the Supreme Court.
The disclosure requirement is expected to apply to explicitly political ads by nonprofit groups for the remaining weeks of the campaign season.
The Supreme Court's decision comes less than a week after a new research report by the government reform group Issue One, which puts some dollar amounts on what these unreported donors are giving. The report, which took a year of research, finds that the top 15 politically active nonprofits raised and spent more than $600 million on campaigns between 2010, when Citizens United boosted secret fundraising, and 2016.
The top four spenders identified by Issue One are the U.S. Chamber of Commerce, the mainstream conservative Crossroads GPS, the Koch network's Americans for Prosperity and the National Rifle Association. Issue One says that collectively, the four groups pumped at least $357 million into elections between 2010 and 2016.