Even as Sen. Bernie Sanders continues to insist he can win the Democratic nomination, several prominent liberals have lined up behind front-runner Hillary Clinton in recent days — signaling that the time is now to begin unifying the party to take on Republican Donald Trump.

In endorsements of Clinton this week, California Gov. Jerry Brown and an influential environmental group, the NRDC Action Fund, argued that Democrats must stop fighting one another over their party’s nomination. Brown wrote in an open letter that Clinton offers the best chance to defeat Trump’s “dangerous” candidacy, while the fund, a political affiliate of the Natural Resources Defense Council, wrote that liberal groups must rally around Clinton because Trump’s policies would “take us back 100 years.”

Both efforts seemed aimed at bolstering Clinton in California, an enormous liberal state that Clinton could lose Tuesday even as she is expected to effectively clinch the nomination in other states. They also reflected how damaging it could be to Clinton to kick off her battle against Trump with such a symbolic defeat.

“This is no time for Democrats to keep fighting each other,” Brown said in a message addressed to California Democrats and independents who will vote in next week’s primary. “The general election has already begun.”

Speaking in D.C. on May 24, Sen. Elizabeth Warren (D-Mass.), slammed GOP presidential candidate Donald Trump, saying the "central question" of the 2016 election is "whether this country works for billionaires like Trump and their big-bank friends or whether this country works for everyone else. " (Sarah Parnass/The Washington Post)

Perhaps the biggest sign that a new effort is underway among liberals to begin healing fissures within the party comes from Sen. Elizabeth Warren (D-Mass.), the heroine of the left. Although Warren still has not endorsed Clinton — and remains the lone Democratic woman in the Senate not to do so — she has stepped up her attacks on Trump, and her advisers have begun communicating regularly with the Clinton campaign.

“We are in regular contact with her team and are very excited about the prominent role she has taken in defining what’s at stake in the election,” said Clinton spokesman Brian Fallon. Both sides continued to insist that there is no formal coordination — and most likely no endorsement by Warren until after the last round of primaries.

The effort is a delicate one given the ardor of Sanders’s most steadfast fans — and their insistence that he can still win the nomination. Sanders is already preparing supporters to reject any pronouncements of Clinton as the party’s presumptive nominee Tuesday, when primary results in New Jersey are expected to bring her total of pledged delegates (those won in nominating contests) and superdelegates (party leaders and elected officials who may back the candidate of their choosing) past the 2,383 she needs to secure the nomination.

At a rally in Santa Cruz, Calif., on Tuesday, Sanders warned that the media is expected to declare “the primary process is over, Secretary Clinton has won.” His statement was met with boos.

“That is factually incorrect. It’s just not factually correct,” the senator from Vermont said, predicting that he would win California and some other states Tuesday and head into the convention with enough momentum to flip the allegiances of superdelegates who have announced support for Clinton.

Voting will also take place Tuesday in Montana, New Mexico and both Dakotas.

Clinton leads in all delegates 2,312 to Sanders’s 1,545, according to a tally by the Associated Press — although that number includes support from a large majority of superdelegates who are free to switch their allegiance at any time, for instance if Sanders pulls ahead in pledged delegates. Clinton leads among pledged delegates 1,769 to 1,501, meaning it is possible, if entirely unlikely, for Sanders to overtake her with landslide victories Tuesday, when 475 pledged delegates are at stake in California alone.

Although Clinton and her advisers remain confident about her prospects for securing the nomination, they have grown increasingly antsy about California. The former first lady canceled an event in New Jersey on Thursday to head to the West Coast sooner. She announced a five-day schedule chock full of campaign stops throughout the state, from Northern to Southern California, including a national-security speech in San Diego aimed at strengthening her position against Trump.

During an interview Tuesday with MSNBC, Clinton did not give any hint of nerves over the outcome of California’s primary, though.

“I’m feeling very positive about my campaign in California,” she said. “We are working really hard.”

Democrats had largely accepted that Sanders was unlikely to concede the race until after the last votes are counted June 14, when the final nominating contest will take place in the District of Columbia.

But the latest rumblings among progressives this week reflect a new level of worry about Clinton’s weaknesses heading into the fall and the need for her to shore up her support in the party’s liberal wing — starting now.

“There are two clear messages: one, that we need to work together, and two, that Hillary Clinton will move forward a populist, progressive agenda,” said Mo Elleithee, a former Democratic National Committee official and a former Clinton aide. “It’s reassuring them that their message and their goal will be heard.”

He added: “Do folks want to be responsible for putting Donald Trump in the White House? Because it could happen.”

The challenge is one that former Vermont governor and presidential contender Howard Dean knows well. He recalled coming to his own recognition of the need to unify after he lost the Democratic nomination in 2004 — and spoke to former vice president Al Gore.

“After my ranting and raving, he just said: ‘Look, it’s not about you. It’s about the country,’ ” Dean recalled. “That’s where Jerry Brown is — it’s where most of us are.”

Dean, who supports Clinton, said that there has been a ferment of calls for unity among progressives so early because of the mathematical near-impossibility that Sanders could overtake Clinton. And, he added, unification will take time.

“They are going to have to get to a place where Hillary Clinton is the candidate that they are going to support, having supported somebody else in a contested, occasionally bitter primary,” Dean said. “And that’s a hard thing to do, to climb down from your position. It takes time.”

It’s a challenge that Brown, who competed against Bill Clinton in the 1992 Democratic primary, also knows well. Brown’s support for Hillary Clinton this week follows decades of sometimes acid criticism of both Bill and Hillary Clinton — including one zinger in which he called Bill Clinton the “prince of sleaze.” In a debate in 1992, Brown raised the issue of Hillary Clinton’s ethics, accusing the then-Arkansas governor of “funneling money to his wife’s law firm for state business.” Bill Clinton famously replied that Brown was “not worth being on the same platform as my wife.”

More than 20 years later, Brown is finally standing with Hillary Clinton, lending the weight of his liberal credentials to her cause.

“It’s not like he’s a longtime Clintonite,” Elleithee said. “This is a guy who is a hero to many progressives, and the fact that he came out and said it is time for us to focus . . . that was a very strong signal to Sanders.”

Still, even in his endorsement, Brown signaled the challenge Clinton faces with liberals. The open letter was lukewarm in its praise for her, and while Brown laid out his reasons for his vote, he did not expressly urge California Democrats to vote for her.

Brown also praised Sanders on his way to calling Clinton qualified and capable.

Anne Gearan and John Wagner in Washington and David Weigel in Santa Cruz contributed to this report.

Correction: An earlier version of this report misstated the name of the Natural Resources Defense Council.