After carefully avoiding any involvement in the Democratic presidential primary, Gov.
"This is no time for Democrats to keep fighting each other," he wrote. "The general election has already begun."
Brown said Clinton has made a persuasive case that she is capable of pushing forward a progressive agenda, and her lead over rival Bernie Sanders is so large at this point that the insurgent Vermonter no longer stands any realistic chance of winning the party's nomination. Clinton is poised to wrap up the nomination on June 7, when California and five other states will be voting.
Still, Brown's endorsement at this stage is yet more evidence of the closely fought primary ahead in California. A recent poll showed Sanders and Clinton in a dead heat in the state, and Clinton cut short a planned campaign swing through New Jersey so she could get back to California by Thursday and hit the stump for several days.
Brown's backing is also an indication of Democratic Party leaders' eagerness to coalesce around their front-runner and kick their general-election campaign into full gear. He wrote that he will be voting for Clinton because "this is the only path forward to win the presidency and stop the candidacy of Donald Trump."
A loss for Clinton in the most populous state in the nation and the last major primary going into the Democratic National Convention in July in Philadelphia would deeply bruise her campaign.
Those close to the governor believe Brown simply thought it was the right time, given his own sense of the campaign's rhythm.
For weeks, Brown had been conspicuously coy about his presidential leanings. In mid-April, the governor said he was "not in any hurry" but reminded reporters that he will serve as a superdelegate to the party's convention.
Even so, it may have been Bill Clinton who helped seal the deal. The former president spent an hour and a half with the governor in Sacramento last week, delaying an evening speech on the campus of Cal State Sacramento.
Helping win an endorsement for his wife from Brown would mark yet another intriguing chapter in one of politics' most tempestuous relationships.
It was Brown, after all, who refused to close ranks after losing to Bill Clinton in the 1992 presidential primary, famously referring to his rival as "the prince of sleaze."
Brown played a role that year not unlike the one Sanders is playing now, running as the outsider against the establishment, demanding the Democratic Party move in a more leftward direction and refusing to yield to the front-runner at a time party leaders were eager for unity.
At the party's 1992 national convention in New York, Brown supporters roamed Madison Square Garden with tape over their mouths, protesting what they said was the muzzling of their candidate by party leaders. They interrupted a speech by Hillary Clinton with shouts of "Let Jerry speak!"
"I've never known Jerry not to speak when he wants to speak," Clinton said at the time. "He's always speaking, near as I can tell."
The uneasiness still had not subsided by the time Brown had launched his campaign for governor, in 2010. His GOP rival at the time,
Brown responded by pointing out that Bill Clinton lied about his philandering in the White House, mocking Clinton's notorious line, "I did not have sexual relations with that woman."
"Clinton's a nice guy, but who ever said he always told the truth?" Brown told a crowd at the opening of a Democratic Party office in East Los Angeles in 2010. "You remember, right? There's that whole story there about did he or didn't he. OK, I did — I did not have taxes with this state."
Brown later apologized. And Bill Clinton ultimately endorsed his gubernatorial bid that year.
The governor worked hard to stay on the sidelines after Hillary Clinton launched her White House bid last year.
But his endorsement, one week before election day, may not have the impact that it could have a few weeks earlier. More than 1.5 million ballots have already been cast through the mail in California, according to an analysis by Political Data Inc., a well-known campaign data firm. A number of other prominent Democrats, from statewide elected officials to most every state legislator, have already spoken up in favor of Clinton.
Sanders' team argued that the endorsement was akin to party leadership panic.
"That may be why he's weighing in now on behalf of the Democratic establishment," said Jane Sanders, the candidate's wife, in a CNN interview on Tuesday.
Veteran campaign watchers in California all but declared that the endorsement would signal the beginning of the end of a raucous race.
"He's really become an elder statesman in the Democratic Party," David Townsend, a longtime party strategist, said of Brown. "I think he realizes that it's his state and that we need to pull together."
Regardless of whether animosity between the two big personalities remains, Trump's agenda could be more disruptive to California than any other state, as Brown alluded to in his open letter.
The presumptive GOP nominee is looking to roll back many of the California policies that Brown's legacy has been built on, particularly those involving rights for migrant workers and combating climate change.
Trump "has called climate change a 'hoax,'" Brown warned in his letter. "He has promised to deport millions of immigrants and ominously suggested that other countries may need the nuclear bomb.
"I want to be sure it is Hillary Clinton who takes the oath of office, not Donald Trump," Brown wrote.
Halper reported from Washington and Myers from Sacramento.
3:28 p.m.: The story was updated with background on Gov. Jerry Brown.