The Supreme Court on Thursday struck down a California law that required nonprofits to hand over a list of their biggest donors.

Why it matters: Some campaign-finance advocates have feared the court will begin chipping away at disclosure rules more broadly, making it harder and harder to figure out who’s funding major political causes.

The big picture: In a 6-3 ruling authored by Chief Justice John Roberts, the court said California had subjected donors to the threat of public harassment and intimidation, undermining their First Amendment right to free association.

Background: California requires nonprofit organizations to give the state a list of their biggest donors each year. The state is supposed to keep that information private, but has routinely failed to do so. Donors’ names and addresses have often become easily available to the public, according to briefs in the case.

• A pair of conservative nonprofits — including Americans for Prosperity, an arm of the Koch brothers’ political empire — sued California. Its pattern of making donor information public put individual donors in physical danger, they argued, especially in this toxic political climate.
• Although conservative organizations brought the suit, the ACLU and the NAACP Legal Defense and Education Fund took their side. The most relevant precedent in this case was set in the 1950s, when Alabama tried to publicly disclose a list of NAACP members as a way to intimidate civil rights activists.

The other side: California said it collected donor information to help investigate potential fraud, but that argument didn’t get very far with the justices.