When Joe Biden sewed up the Democratic nomination in April, the liberal rebellion in the party went quiet as its leaders rallied behind Biden with remarkable speed, fearing discord would damage his chances against President Trump.

But now, as Biden has built a lead over Trump in the polls and the nation confronts a once-in-a-generation upheaval, liberals are responding with a renewed attempt to push the party left. They are making forceful demands of Biden and increasingly embracing leftist candidates down the ballot, awakening the prospect of divisions that party leaders thought they had squelched.

A group of mostly white liberal activists, for example, is aggressively pushing for Biden to choose Sen. Elizabeth Warren (D-Mass.) as his running mate, challenging a call by African American leaders for Biden to choose a black woman. Sen. Amy Klobuchar (D-Minn.) took herself out of the running in recent days amid criticism from many activists of her record as a prosecutor.

Liberals have grown increasingly vocal in pressing Biden to take a harder line against law enforcement, frustrated he and other Democratic leaders have flatly rejected calls to “defund the police.”

And a big test comes Tuesday. Socialist stars such as Sen. Bernie Sanders (I-Vt.) and Rep. Alexandria Ocasio-Cortez (D-N.Y.) have endorsed left-leaning hopefuls in Democratic congressional primaries — including contests this week in New York and Kentucky — against candidates backed by party leaders.

“I think the progressive left is just trying to figure out wherever we can get a toehold,” Ocasio-Cortez said in an interview.

The Democatic splits have grown more complicated in recent weeks, rooted in debates over race, ideology and class. Activists are pushing party leaders to do more to seize a moment that’s seen historic demands for reform and a pandemic that has devastated poor and nonwhite communities. Leftists who never warmed to Biden but laid low after his victory are finding new channels for their energy.

“There are a lot of problems, and there is an urgent need for change,” said Rebecca Katz, a liberal strategist and adviser to Jamaal Bowman, who is challenging longtime Rep. Eliot L. Engel (D-N.Y.). “What you’re seeing is there are not enough people in power — enough Democrats in power — who are fighting for that change.”

Still, it’s unclear how effective the push will be, given the divisions in the liberal movement and the few solid wins under its belt. The movement itself is complex and sometimes fractured; many white liberals who focus on economic issues, for example, do not always see eye-to-eye with the black activists leading the current street protests.

Biden has taken a nuanced approach to the left. He heaps praise on liberal leaders and causes, and he has moved in their direction on such issues as health care by lowering the age for Medicare eligibility. But he has steadfastly refused to adopt more provocative ideas — including Medicare-for-all and defunding the police — that could prove a harder sell in the general election.

At a fundraiser Friday, Biden echoed the language of liberal activists, saying the country is at “one of those inflection points” where “fundamental systemic change” is possible. He compared the moment to the time of Bull Connor, the Birmingham, Ala., official who unleashed violence against the Freedom Riders in the 1960s.

Since 2016, Democrats have desperately feared a repeat of that year’s bitter split between backers of Sanders and Hillary Clinton, which many believe helped Trump win. The left’s renewed willingness to publicly take on Biden suggests a relaxation of those anxieties.

Many activists say their goal is to turn this moment’s historic energy into influence in the run-up to the election — and beyond, should Democrats win the White House and the Senate. That has given new life to a movement that was dispirited after the once-promising presidential campaigns of Sanders and Warren faded.

“There were a few weeks there where we were kind of going through reflection and diagnosis,” said Bill Neidhardt, who helped lead the Sanders campaign’s operations in Iowa and Virginia. “The important part was to come out of that with either a really clear plan or a fresh sense of energy. And I do think that has happened.”

But channeling the energy of the movement has been a challenge for liberals in recent years, due in part to internal splintering. Many supporters of Warren and Sanders retain icy relations after a bruising primary. As some activists are criticizing Biden, other liberal leaders such as Sanders are still giving him positive marks. And the protesters now seizing the country’s attention do not fit neatly into preexisting liberal groups.

That’s making it tougher to do things like topple centrist Democratic incumbents, said Cornell Belcher, a Democratic pollster who worked for Barack Obama. “The hard progressive left, or hard liberal left, has not shown the ability to be as organized as we’ve seen the hard conservative right at taking out Republicans in primaries,” Belcher said.

The divisions were evident recently in the competition to be Biden’s running mate. More than 100 activists, leaders and celebrities from the left, most of them white, signed a letter urging Biden to pick Warren. It came as many black leaders were pressuring Biden to choose an African American, saying to do otherwise in the current moment would be an affront to a long-persecuted community.

The liberals’ letter contended, among other things, that Biden was already strong with voters of color and needed an ambassador to Sanders backers who have not yet warmed to him.

Laurence Tribe, a prominent law professor at Harvard who signed the letter, told The Washington Post that while he is sensitive to the push for a black woman to join the ticket, “I think African Americans above all would be the first to say they are more interested in results than cosmetics.”

Several African American Democrats said they found the remark offensive. “That statement was very callous,” said Nina Turner, who was a national co-chair on the Sanders campaign. Belcher said he found the letter “so condescending” and that Tribe’s remark “set a lot of people off.”

Hours after his comments were published, Tribe said he was sorry. “I apologize for my choice of words,” Tribe tweeted. He added in a subsequent tweet, “I’ve never doubted that racial identity is a significant variable in American governance. It should count heavily in favor of previously excluded groups as part of a person’s full record of background, skills, and values. I’m FOR Warren, not ANTI-excellent others.”

White and black liberals are more united in pressuring Biden to embrace a more sweeping overhaul of the criminal justice system. More than 50 liberal groups signed a letter to Biden recently criticizing his response to the protest movement and panning his promise to add $300 million for community policing programs.

Angela Rye, a civil rights leader and former executive director of the Congressional Black Caucus, said advocating those funds “when people are calling for resources to be divested from police departments is not just tone deaf, it’s irresponsible.” Rye also took issue with Biden’s plan for black America, titled “Lift Every Voice,” saying the blueprint is full of “platitudes.”

Lenard Larry McKelvey, who goes by the name Charlamagne tha God as co-host of “The Breakfast Club,” a radio show popular with black audiences, said early this month that the Lift Every Voice plan was “weak on atonement.” McKelvey said Biden must go further than calling for a study on reparations. He also called on Biden to embrace legalizing rather decriminalizing marijuana.

After interviewing Biden last month, McKelvey said he was left with the impression the candidate did not even recognize the name of his own proposal.

More broadly, the turmoil of recent weeks — a surge of activism not seen since the 1960s — has revived the sense among some on the left that Biden is too wedded to the past to grasp the moment’s potential.

“Joe Biden is somebody who very much came up in the politics of the last 40 to 50 years, and I think what we’re seeing right now in the streets of this country and in the rejection of establishment politics is a rejection of the status quo,” said Evan Weber, co-founder of the Sunrise Movement, a group of youthful climate activists.

A Biden campaign official said the team is listening to activists and that all factions of the party are unified by a strong desire to defeat Trump.

“We understand the sentiment of what some activists are saying,” said Symone Sanders, a senior Biden adviser. “We hear, we share their anger, we hear their pain. And I think we all have the same goals, frankly, in terms of stopping this horrible violence and undoing systemic racism.”

Biden has adapted in some ways to a landscape that has shifted dramatically since he sewed up the nomination. He delivered a speech in Philadelphia that received widespread praise in the party, embracing a ban on police chokeholds and calling for a national use of force standard. He has also begun sketching out a more transformational agenda to address public health, economic and racial problems.

“This is not a man who won the Democratic nomination and then asked the progressive movement to bend the knee to him,” said Michigan Lt. Gov. Garlin Gilchrist (D), who is black and voted for Sanders in 2016. “This is a person who made an active decision to govern in coalition with the entirety of the Democratic coalition, and that includes the progressive movement that is calling for big, bold change.”

Beyond the presidential race, Democrats face an array of heated congressional primaries, including a high-profile race in New York’s 16th Congressional District on Tuesday where Bowman, a former middle school principal, is trying to unseat Engel, a committee chairman and three-decade House member.

Bowman, who is black, recently won the support of Sanders, Ocasio-Cortez and Warren. Engel, who is white, is supported by New York Gov. Andrew M. Cuomo, Senate Minority Leader Charles E. Schumer (D-N.Y.) and former secretary of state Hillary Clinton.

The challenger has accused Engel of being absent from the district during the pandemic and has voiced support for calls to defund the police.

Engel recently drew criticism after he repeatedly asked to speak at a Bronx news conference on racial justice, then said, apparently not realizing the microphone was live, “If I didn’t have a primary, I wouldn’t care.”

In a recent debate, Engel called himself “one of the real pains in the neck” to President Trump, stressing his participation in the president’s impeachment.

A Senate primary in Kentucky, for the right to face Senate Majority Leader Mitch McConnell (R), has also caught the attention of the left. Charles Booker, an African American state representative who has raised his profile amid the protests, is seeking an upset over Amy McGrath, a white former fighter pilot favored by many Democratic leaders.

Like the New York race, the contest pits Schumer, who backs McGrath, against Sanders, Warren and Ocasio-Cortez, who are supporting Booker. McGrath has long been the favorite, but rising anger over the police shooting of Breonna Taylor, who Booker has said was a family friend, has increasingly become a point of focus in the race.

“Only a movement can beat Mitch McConnell,” Booker tweeted Saturday. He added, “We were so invisible to him, he never saw us coming.”

David Weigel in the Bronx contributed to this report.