Some of the most scathing, if veiled, admonitions came Sunday from Sen. Bernie Sanders, I-Vt., who repeatedly condemned "middle ground" policy ideas and appeared to criticize Biden for not attending a Democratic gathering where Sanders and 13 other White House hopefuls spoke.
"We cannot go back to the old ways," Sanders warned in a speech at the California Democratic Party convention here, as supporters cheered and waved signs bearing his name. "We have go to go forward with a new and progressive agenda."
On Saturday, Sen. Elizabeth Warren, D-Mass., delivered a fierce rebuke of Biden's professed faith that Republicans will be more cooperative once President Donald Trump is out of office. The crowd showered her with loud cheers. Before Warren spoke, House Speaker Nancy Pelosi, D-Calif., faced shouts from the hometown audience demanding that she and other Democratic leaders move to impeach Trump.
The broadsides amounted to the loudest warning yet from the party's left flank that it intends to aggressively challenge the centrist ideas and cautious politics that have gained a foothold eight months before presidential balloting begins with the Iowa caucuses. After signs of promise earlier this year, when Sanders topped the polls and Rep. Alexandria Ocasio-Cortez, D-N.Y., quickly asserted her influence on Capitol Hill, the liberal movement has stagnated.
Biden, a more traditional Democrat who has rejected Sanders' brand of socialism and calls for dramatic change, has built a persistent and comfortable lead in the polls. Medicare-for-all, which once seemed poised to become a dominant proposal, has met resistance in the party. And Pelosi has rebuffed calls to open impeachment proceedings against Trump, despite fresh questions about his conduct raised by now-former special counsel Robert Mueller in a recent public statement.
Leftist candidates and activists have concluded that they can no longer wait for Biden to fade on his own, as some once hoped. Nor can they count on Democratic congressional leaders to fight Trump with all the tools at their disposal, without some prodding.
"Now is not the time to play it safe," said Kacey Carpenter, 56, who came to see Sunday's speech dressed in a Sanders shirt and hat. "If we play it safe, Trump is going to get reelected - or worse."
But the power of their newly emboldened movement remains unclear. Even here in deep blue California, it faces hurdles. Late Saturday, the state party overwhelmingly elected a labor leader from the mainstream ranks of the party as its new chairman. He defeated a liberal activist backed by many Sanders supporters.
And interviews with current and former elected officials, strategists and donors in California revealed a relatively high level of confidence in Biden's ability to defeat Trump. Many Democrats see that as the most important quality in a candidate.
"This is not a normal presidential election," said former Sen. Barbara Boxer, D-Calif., a who called defeating Trump a "moral imperative."
Asked who fits that bill, she replied, "I think the only one right now is Joe." That could change, said Boxer, who has not endorsed a candidate.
The California convention kicked off a new phase of the race in which Democratic voters will have more frequent opportunities to size up the candidates on the same stage. The first televised debates will be held in late June, with more to follow in July and September. Candidates are also scheduled for myriad joint events in coming weeks.
On policy matters, Sanders has been one of the most vocal Biden critics in the crowded field, frequently comparing his opposition to sweeping trade deals and the Iraq War with Biden's support for those things. Yet he seemed to open a broader and more personal contrast with Biden on Sunday.
"There is a debate among presidential candidates who have spoken to you in this room - and those who have chosen for whatever reason not to be in this room - about the best way forward," Sanders said, appearing to single out Biden for not showing up.
The former vice president, who has largely avoided engaging his Democratic rivals in favor of focusing on Trump, was in Ohio on Saturday to deliver a speech at a dinner hosted by the Human Rights Campaign, a group that advocates for LGBTQ rights. Biden accused Trump of "callously extending his power over the most vulnerable," and he vowed to swiftly pursue an anti-discrimination bill that passed in the House if he is elected president.
After some booing in the convention crowd Sunday in response to Biden's absence, acting state Democratic Party Chairwoman Alexandra Gallardo-Rooker told delegates that Biden called her last Wednesday to say he was committed to attending the HRC dinner. "I get it," she said in defense of Biden, adding that he will be in California "many times."
Sanders repeatedly told the crowd that gathered for his remarks - which was smaller than the session Saturday, when 11 candidates spoke - that there could be "no middle ground" when it comes to winding down foreign wars, lowering prescription drug prices and enacting universal health care, among other priorities.
The differences between liberals and centrists - epitomized by Sanders and Biden - on major issues are substantive. On health care, for example, Biden supports expanding the current system to allow all Americans to buy into Medicare. Sanders supports Medicare-for-all, a mandatory universal program.
Once the leading candidate in the polls, Sanders is now running a distant second to Biden. He has struggled to attract new supporters and prevent old ones from fleeing to other camps. Amid these setbacks, his allies have sharpened their attacks on the party establishment, reviving a core theme of his 2016 campaign against Hillary Clinton.
The memories of that campaign were still fresh at a Sanders fundraiser in San Francisco on Saturday - his first in-person fundraiser of the campaign, where admission started at $27 - when he often brought up his run in the previous cycle.
But he is not the reflexive alternative to Biden; Warren's criticism was equally pointed as she sought to push more liberal positions. The similarities between Warren and Sanders served as reminders that while leftists are effectively declaring war on Biden, they are also at war with themselves.
Warren, who has improved her standing in recent polls, delivered a standout speech on Saturday that drew some of the loudest cheers of the convention - louder even than those awarded to home state Sen. Kamala Harris, D-Calif., who is also running for president.
"Some say that if we just calm down, the Republicans will come to their senses. But our country is in a crisis. The time for small ideas is over," Warren said. Her words were seen a dig at Biden, who has vowed to work with Republicans he insists are fundamentally different from Trump.
Other presidential candidates who spoke at the convention on Saturday also sought to present an alternative to Biden. Like Sanders and Warren, they didn't mention the former vice president by name.
"The riskiest thing we could do is try too hard to play it safe," said South Bend, Indiana, Mayor Pete Buttigieg.
"There is no back to normal," Buttigieg said.
"We don't need a crime bill, we need a hope bill," said Rep. Eric Swalwell, D-Calif., apparently making reference to a 1994 bill Biden championed that has become controversial.
Harris, who is in intense competition with Biden for support from black voters, did not attack him or his policies. Asked Saturday what she made of his absence from the convention, Harris shrugged.
In many ways, Harris epitomizes the part of the Democratic Party caught between its liberal section and its moderate side. While she has been criticized for some past centrist positions, in her Saturday speech she made a point of emphasizing her support for starting impeachment proceedings.
The fight between liberals and centrist Democrats will play out throughout the nation and in California. A virtual lock for Democrats in a general election, the state has expanded its reach into the nomination fight by moving its primary up to March 3, on which a constellation of big states will cast votes after February's balloting in a series of smaller environs.
"We always get attention because of our donor class," said Los Angeles Mayor Eric Garcetti, a Democrat who after sidelining a presidential candidacy of his own has spoken to nine candidates as he maintains a role as an uncommitted broker. "But now, our voters are as valuable as Midwestern, Southern, Northeastern voters and in many ways we are the ultimate prize."