WASHINGTON – The Supreme Court ruled on Thursday that charities cannot be compelled to tell state regulators the names of their biggest donors, a requirement that some charities say would hold back contributions.

The 6-3 decision was a victory for conservative groups led by Americans for Prosperity, backed by billionaire industrialist Charles Koch and his late brother, David. They said a California disclosure requirement violated their First Amendment rights and exposed major donors to intimidation or potential attack.

Along with the Thomas More Law Center, a conservative Christian legal organization, the Koch Group challenged a state law requiring charities to file annual reports and include a copy of an IRS document listing names and addresses of their main contributors.

California said it collected the information to help detect the use of charities to commit financial fraud. Filings are meant to be kept private, but some of them ended up being posted online. Lawyers for the state have said it has tightened the rules for handling donor information, making data leakage unlikely in the future.

When the Supreme Court was faced with the same issue in 1958, it struck down an Alabama law requiring the NAACP to disclose the names of its members in the state. The tribunal then said the measure would expose members “to economic retaliation, loss of employment, threat of physical coercion and other manifestations of public hostility.”

In this case, the two conservative groups have attracted a wide range of supporters. The Chamber of Commerce and the National Taxpayers Union Foundation were among those who filed friend of the court briefs on their side, as were the American Civil Liberties Union, the Electronic Frontier Foundation, and the Committee on American-Islamic Relations. .

Supporters of controlling money spent on political campaigns urged the court not to use the case to tamper with federal laws requiring political campaigns to disclose their contributors. The court also ruled that supporters of state referendum votes were prohibited from keeping their names a secret.

In the past, the court has said that any concerns about the exposure of political contributors were outweighed by the government’s interest in enforcing campaign finance laws.