And while Brown was no fan of Trump, the former governor contended that compelling the release of tax returns could be unconstitutional, and cautioned that signing the bill would launch a political standoff into unknown territory, like requiring candidates’ birth certificates, health records or report cards.

“It’s really going to gin up the Trump base, which could affect congressional candidates and legislative candidates [in California],” said Dana Williamson, a former political adviser to Brown. “From a purely political standpoint, the benefits don’t outweigh the risks.”

Newsom rose to the governorship after spending two terms as California’s lieutenant governor — a position that’s elected independent of the governor, not appointed. While his office was mere feet from Brown’s, the wily veteran Brown and the ascendant young Newsom had a famously chilly relationship.

Newsom has until Tuesday to decide.

He is still weighing his next step, saying this week that he was “deeply analyzing” the bill. Speaking on the sidelines of a National Governors Association meeting in Salt Lake City, Newsom said the measure’s constitutionality was “an open-ended question.”

“Some may see it as symbolic. I think it now appears to be much more substantive,” Newsom told POLITICO. The legislation is focused on primaries, so, he said, “at the end of the day, the president will end up on the November ballot regardless, and the likelihood of the taxes being released is questionable.”

A spokesperson for state Sen. Scott Wiener (D-San Francisco), a co-author of the bill, said in a statement to POLITICO that they had “made the case about why this is both legal and good policy” but “haven’t received any indication from the Governor about whether he will sign the legislation.”

California has sued the Trump administration dozens of times under Newsom, and prior to becoming governor, Newsom publicly pushed Trump to release his tax returns and insinuated that Trump was “hiding” potentially damaging details about his sprawling business empire.

“Politically, this is a winner for Gavin Newsom,” said Nathan Ballard, who served as Newsom’s press secretary when he was mayor of San Francisco. “He’s established himself as one of the leading voices against Donald Trump in the United States, and this will only further his reputation as someone who’s willing to stand up to President Trump and the aroma of corruption that surrounds him.”

Despite his animosity toward Trump, Newsom has also sought to cultivate a working relationship with his administration around issues like disaster aid. A direct shot at Trump’s tax returns could torpedo that relationship, but Ballard said Trump “will never do any favors for California” regardless.

“Trump is already going to try to shortchange us in any way he can,” Ballard said. “Trump plays to his base and to his base only, so the only states he’s doing favors for are the ones on his electoral map.”

But there are risks. State Senate Judiciary Chairwoman Hannah-Beth Jackson (D-Santa Barbara) said she didn’t think it would have much effect on either the Democratic or Republican bases within California. “It’s really more a question of, does he view this as something that will impair the state of California’s relationship with this White House, or does he think it’s more important than any one president or any one White House?”

She also pointed out that this year’s bill extends disclosure requirements to gubernatorial as well as presidential candidates. “I think adding the governor further justifies it and takes away a little bit of this sort of partisan prism through which this bill is being viewed,” she said.

Jack Pitney, a professor of politics at Claremont McKenna College who renounced his Republican registration in response to Trump’s candidacy, said if the bill does become law, it could energize Newsom’s opponents by serving as “a long-awaited rallying cry for Republicans who desperately need something to jump-start the party.” But given the likelihood of a legal challenge, he said, that possibility is remote.

“It’s kind of a freebie. He takes credit for taking a stand against Trump, knowing the courts will strike it down,” Pitney said.

A similar battle to pry open Trump’s tax records is playing out in Washington, where the president last week sued to block House Democrats from subpoenaing Trump’s financial records — indicating that Trump would almost certainly challenge California’s law in court. As part of the same litigation, Trump sued New York officials, who recently enacted a law that could give House Democrats access to his state tax returns.

Arguments against the bill did not deter Democratic lawmakers who were determined to use their sweeping control of California government to rebuke Trump, nor did they faze California Secretary of State Alex Padilla, a Democrat and vocal Trump critic who supports the bill.

The measure passed the Legislature with near-unanimous Democratic support, despite Republicans castigating it as unconstitutional grandstanding. While the GOP has withered into near-irrelevance in California, the state is still home to 4.7 million registered Republicans.

“I encourage Democrats in the Legislature to drop the political posturing and focus on addressing the myriad of problems truly impacting Californians, including the skyrocketing cost of living or our failing education system,” California Republican Party Chairwoman Jessica Patterson said after the bill passed.

While Trump is unlikely to pick up California’s electoral votes regardless of whether he is on the ballot, the potential ripple effects elsewhere could impact the larger presidential contest. Officials in other states would closely watch a legal challenge to gauge the viability of following suit, Ballard predicted.

“If the chances for survival look good in court, other states could pick up the baton and enact similar laws, which could have a serious effect on the outcome of the 2020 election,” he said.

David Siders contributed to this report.