Private sector unions can’t use fees paid by nonmembers to fund their lobbying efforts, the National Labor Relations Board ruled.

Unions’ lobbying activities fall outside of their core function to represent workers regarding the conditions of their employment, the NLRB’s Republican majority said in its March 1 decision. Unions can only charge nonmembers “agency fees” for collective bargaining expenses and other nonpolitical activities.

The board ruled that United Nurses and Allied Professionals unlawfully spent agency fees on lobbying state legislatures in Rhode Island and Vermont. The ruling stems from a 2010 unfair labor practice charge filed by Jeanette Geary, who...