U.S. District Judge Morrison C. England Jr. issued a written opinion Tuesday saying the law likely violates the U.S. Constitution.

England said in September that he would temporarily block the law that requires candidates for president or governor to file copies of their personal income tax returns with the California secretary of state's office.

England wrote that the state's concerns about seeing elected officials' tax returns are "legitimate and understandable."

But he said the court's job is to rule on the law's constitutional merits, not whether it is good policy or makes political sense.

A spokesman for Secretary of State Alex Padilla said his office is reviewing the ruling.

Mr. Trump has also been fighting off a legal challenge to release his tax returns in New York. Manhattan District Attorney Cy Vance said last week that he will not seek to enforce a subpoena for the tax returns until October 7, two business days after a judge rules on Mr. Trump's challenge.

Federal prosecutors have joined Mr. Trump in challenging New York's subpoena, saying the president has raised "weighty constitutional issues" in his attempt to stop the subpoena.