The mayor of San Diego has a First Amendment right to advocate for changes to the city’s retirement plan without first meeting with labor unions, the Pacific Legal Foundation said in a brief filed with the U.S. Supreme Court.

The conservative public interest law firm wants the justices to undo a 2018 decision by the California Supreme Court holding that the state’s “meet-and-confer requirements” prevented Republican Mayor Jerry Sanders from championing a citizens’ initiative to switch new city hires to...