Forty-seven years ago, California’s voters opened the state’s presidential primaries to all nationally recognized candidates. That ballot measure could determine the fate of a new state law requiring President Trump and his competitors to release their tax returns to run in next year’s primaries.

California’s presidential primaries used to be friendly territory for “favorite sons” who weren’t running national campaigns. In 1960, when John F. Kennedy was elected president, the only major candidate on the state’s Democratic primary ballot was Gov. Pat Brown. And Richard Nixon’s 1968 election was preceded by a Republican primary in his home state in which the only listed candidate was Gov. Ronald Reagan.

So in 1972, 61% of the state’s voters approved Proposition 4, a state constitutional amendment, which said the presidential primary ballot must list “recognized candidates throughout the nation or throughout California,” as determined by the secretary of state. Now the state Supreme Court has agreed to decide whether Prop. 4 prohibits California from requiring Trump to disclose five years of tax returns to appear on next year’s Republican primary ballot.

The 1972 measure was intended to “guarantee California voters the right to consider all of the candidates seeking election to the highest office in the land,” lawyers for the state Republican Party and its chairwoman, Jessica Millan Patterson, said in a filing with the court.

They said the new state law on tax returns, sponsored by Democrats and signed by Gov. Gavin Newsom, would prohibit Secretary of State Alex Padilla “from exercising his constitutionally delegated duty to place the name of all nationally recognized presidential candidates” on the ballot.

Padilla, a Democrat who is the state’s top elections official, viewed Prop. 4 differently.

The ballot measure merely tells the Legislature to “pass laws that ‘provide for’ primary elections,” state lawyers representing Padilla told the court. “It imposes no mandatory duty on the secretary of state, but rather provides that the secretary will ‘find’ candidates that are ‘recognized ... throughout the nation’ and include them on California primary ballots.”

At the same time, Padilla’s lawyers said, “California has a vital interest in ensuring that its citizens are an informed electorate,” and tax returns can inform voters about a candidate’s “financial status and honesty concerning financial matters.” Trump is the first president in more than four decades to refuse to make his tax returns public.

In a prompt response signed by all seven justices, the court agreed to review the Republican lawsuit and set a tight briefing schedule over the next two weeks, with a hearing to follow shortly. The court ordered both sides to discuss the history and meaning of Prop. 4 and any related legislation, as well as any standards used by the secretary of state “to assess who is a ‘recognized’ candidate.”

Clearly, the justices think the Republican lawsuit’s reliance on Prop. 4 “is not a frivolous argument,” said Richard Hasen, a UC Irvine law professor and election law specialist.

Jessica Levinson, a professor at Loyola Law School in Los Angeles who specializes in election law and governance, said there appear to be legitimate arguments on both sides.

While Prop. 4’s goal was “opening up the election,” she said, Padilla rightly argues that he and the state still have some discretion over which candidates to include. “They can still control access to the ballot,” she said. “You still need a certain number of signatures and a filing fee, or maybe release your tax returns.”

On the other hand, Levinson said, the tax return requirement “goes a little further than what most people understand to be laws that inform the electorate.” And at least some of the justices are likely to take into account modern-day lawmakers’ motives in “targeting one presidential candidate,” she said.

The case is separate from five suits filed in federal court — one by Trump, three by his allies and the fifth by a would-be Republican challenger, Rocky De La Fuente — saying the California law violates the U.S. Constitution by creating a new qualification for presidential candidates. U.S. District Judge Morrison England of Sacramento has scheduled a hearing Sept. 19 on the plaintiffs’ motions for an injunction against the state law.

But the federal case is almost certainly headed for higher courts. The California court is the highest authority on the meaning of the state Constitution, and a ruling in the Republicans’ favor would scuttle the state law without any need for further action in the federal lawsuits.

It comes before a court with four justices appointed by former Democratic Gov. Jerry Brown and three by Republican governors, but also a court that decides a majority of its cases unanimously. The state justices’ dissenting opinions rarely contain the ideological fervor commonly shown by dissenters on the U.S. Supreme Court.

On the other hand, the current California court has not considered another case with the political consequences of the tax return dispute.

These justices, Levinson said, appear to be “uncomfortable with decisions that look politically motivated.”

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @BobEgelko