(CNN) California Governor Jerry Brown lent his support to Hillary Clinton Tuesday, saying it was the only way to "stop the dangerous candidacy of Donald Trump."

Next week's California primary is the final battleground for Clinton as she prepares to clinch the nomination. But it has become an unexpectedly tight race between her and Bernie Sanders, which does not bode well for Clinton.

Even though she will likely secure the nomination in New Jersey, which also goes to the polls June 7, Clinton has banked on the support of non-white Americans. A loss in the diverse Golden State would show that Sanders has made in-roads with these voters. Sanders has pledged to stay in the race until the Democratic convention in July.

In an open letter to Democrats and independents in California, Brown said he was "deeply impressed" by how well Sanders has done and with his message of the growing divide between the wealthy and the rest of Americans.

But Clinton knows how to get things done and advance the Democratic agenda, the governor said. He pointed out that she has approximately 3 million more votes and hundreds more delegates than her rival. Also, she needs to win only 10% of the remaining delegates to clinch the nomination, he said.

"Clinton's lead is insurmountable and Democrats have shown -- by millions of votes -- that they want her as their nominee," Brown wrote.

"The stakes couldn't be higher. This is no time for Democrats to keep fighting each other," he continued. "The general election has already begun. Hillary Clinton, with her long experience, especially as Secretary of State, has a firm grasp of the issues and will be prepared to lead our country on day one."

Brown also listed his fears of what a Trump presidency could mean, pointing out the Republican frontrunner has called climate change a hoax, promised to deport millions of immigrants, suggested other countries need nuclear weapons and promised to nominate conservative justices to the Supreme Court.

Bernie Sanders' wife, Jane Sanders, dismissed the endorsement Tuesday afternoon as another sign of the "Democratic establishment" ganging up on the Vermont senator.

"Throughout the country, Bernie has had to take on the governors, the senators, the congressmen, the Democratic establishment in each race," Jane Sanders told CNN's Wolf Blitzer on "Wolf." "He's won 20 states and there's a number of them up this coming week that I think he'll win as well, and California might well be one of them, and that may be why (Brown is) weighing in now on behalf of the Democratic establishment."

Tuesday night, the Vermont senator responded to Brown, saying part of his campaign was about taking on the Democratic establishment.

"I really have not heard what the governor has to say and I like Jerry Brown, but people can make their own choices," Sanders said, adding that he believes he will win California.