People are in the streets, confronting injustice and demanding fundamental change.

It is the kind of moment that Senator Bernie Sanders spoke about on the 2020 presidential campaign trail, and for decades before that. But when the revolution finally came, it wasn’t his.

The rise of revolutionary sentiment, like many things, is about timing. The coronavirus outbreak had already renewed support for progressive policy proposals, including “Medicare for all.” But the disproportionate impact of the coronavirus on black Americans, combined with the galvanizing death of George Floyd at the hands of the police in Minneapolis, have heightened the call to address systemic racism and police brutality, uniting Democrats — and the country — in a campaign for action in a way that Mr. Sanders’s message of economic equality did not.

“People are sick and tired of police murders of African-Americans,” Mr. Sanders said in an interview. “People are saying enough is enough.”

Mr. Sanders, whose slogan on his campaign was “Not me, us,” described the protests as a validation of his theory of social change: “What I have said for a very long time is that real change is never going to come from the top on down, it’s always from the bottom on up.”

But during his presidential bid, Mr. Sanders at times seemed uncomfortable speaking overtly about race. At a presidential forum in April 2019 for women of color, he offered few specific policy details, and drew some groans from the audience when he referred to marching with the Rev. Dr. Martin Luther King Jr. in response to a question about how he would handle current challenges.

And more recently, during an event in Flint, Mich., in March that campaign aides had billed as an opportunity for him to speak directly to black voters, he decided not to deliver a planned speech and instead largely ceded the stage to panelists including the academic Cornel West.

When Mr. Sanders spoke about racial equality, it was often in the context of economic equality, championing proposals and prescriptions that he believed would improve the lives of all working Americans. He said that policies like single-payer health care would address higher maternal and infant mortality rates in black communities. And he wanted to legalize marijuana and end cash bail, policies he said were aimed in particular at helping black Americans and other people of color.

These proposals, however, also amounted to an implicit expectation that voters trust the government — an especially difficult sell for those including older black voters who feel they have been historically let down by the government.

They were shortcomings that help explain why Mr. Sanders lost to former Vice President Joseph R. Biden Jr. in the Democratic primary race: Unable to win over older black voters, he came in a distant second to Mr. Biden in South Carolina, then went on to lose to Mr. Biden in every Southern state on Super Tuesday. Those defeats, Mr. Sanders’s allies say, contributed to the perception that Mr. Biden was more electable and would fare better against President Trump in the general election in November — a notion that helped propel Mr. Biden to victory in the primary.

Now, some of the same progressive leaders who never quite figured out how to mobilize such a broad coalition in the primary are considering how to not only support the movement, but also harness its energy as they look toward November.

Many Sanders allies maintain that racial justice was and continues to be a central tenet of his ideology. “If you do parse back the things that our campaign was fighting for, those policies, if they were able to come to life, would have certainly changed the positions for African-Americans in this country,” said Nina Turner, a national co-chair of the Sanders campaign and one of his most prominent black surrogates.

Though she praised his agenda regarding black Americans, she conceded that he did not articulate it forcefully enough. “It had that kind of tone to it, but it wasn’t as piercing as this moment demands,” she said.

Yet amid a national movement for racial justice that took hold after high-profile killings of black men and women, there is also an acknowledgment among some progressives that their discussion of racism, including from their standard-bearer, did not seem to meet or anticipate the forcefulness of these protests.

Kimberlé Crenshaw, the legal scholar who pioneered the concept of intersectionality to describe how various forms of discrimination can overlap, said that Mr. Sanders struggled with the reality that talking forcefully about racial injustice has traditionally alienated white voters — especially the working-class white voters he was aiming to win over. But that is where thinking of class as a “colorblind experience” limits white progressives. “Class cannot help you see the specific contours of race disparity,” she said.

Many other institutions, she noted, have now gone further faster than the party that is the political base of most African-American voters. “You basically have a moment where every corporation worth its salt is saying something about structural racism and anti-blackness, and that stuff is even outdistancing what candidates in the Democratic Party were actually saying,” she said.

Already, a split has emerged in the way progressive leaders and protesters approach systemic racism and police reform, raising broader questions about whether elected officials are in sync with what is happening on the ground. While some activists have embraced the protesters’ rallying cry to “defund the police,” many progressive leaders, including Mr. Sanders, are calibrating their approach.

Unlike during the primary season, when he often took the most leftward position, Mr. Sanders has disagreed with protesters’ demands to eliminate funding for police departments, staking out a careful position on police reform.

“Anyone who thinks that we should abolish all police departments in America, I don’t agree,” Mr. Sanders told The New Yorker. In keeping with his stance when he was mayor of Burlington, Vt., he supported paying police officers more.

At the same time, progressive organizations like the Sunrise Movement, a youth-led liberal environmental group that endorsed Mr. Sanders in the primary, have aggressively pushed to defund the police, adopting the policy as one of their own. When Mr. Biden released a statement last week that took a more cautious position on police overhaul, the Sunrise Movement denounced his stance on Twitter. “@JoeBiden you’re hurting any chance you have at defeating Trump by taking these centrist stances,” the group said. “We need someone fighting with us to create bold change, not someone to maintain the status-quo #DefundPolice.”

But while most progressives might not have seen this revolution coming, they are catching up.

Rahna Epting, the executive director of the progressive group MoveOn, said the protests were a time for national groups like hers to listen to the grass roots. “In terms of what we do, we see the people on the streets right now, this is completely organic,” she said. “This is beyond any one organization or institution.”

She added: “We’re recognizing the moment is not ours, it’s the people’s, and we need to flank the people right now.”

The protests are not directly connected to partisan politics, even though there are some similarities between their broad demands and the revolutionary sentiment embodied by Mr. Sanders’s campaign. But if there is overlap, it is not yet clear whether the energy on the ground, particularly among young progressives who supported Mr. Sanders but remain dissatisfied with Mr. Biden, will translate to enthusiasm at the ballot box in November.

Progressives at both the national and grass-roots level are still trying to push Mr. Biden to the left even as he has begun to adopt the language of systemic disruption. His willingness to satisfy their demands — on policing perhaps most urgently but also on issues like climate change and health care — could help determine whether he is successful in the general election.

Despite persistent ideological disagreements, some progressive leaders are optimistic that the mass social movement will become an animating force in the upcoming election, especially for voters on the left who may have been unhappy initially with Mr. Biden. In a Pew Research Center survey released last week, 91 percent of Democrats and those who lean Democratic said they supported the Black Lives Matter movement.

“At some point, many of the people on the street will view this election as a referendum on black lives,” said Maurice Mitchell, the national director of the left-wing Working Families Party and a leader in the Movement for Black Lives, a coalition of rights groups. In addition to elevating the demands of protesters, he said, his group plans to support candidates who are “brave enough to say that this is a time that we take on the police.”

Some progressives point to the demonstrations occurring around the country, in big cities and small towns, as proof that many Americans did support the idea of systemic, revolutionary change, even if it did not succeed as an electoral argument for Mr. Sanders in the primary.

“We see on television wonderful evidence of the kinetic energy of the Democratic Party,” said Faiz Shakir, who served as Mr. Sanders’s campaign manager, “that was obviously evident during the Bernie Sanders campaign, too.”

Many political observers are eagerly awaiting the results of the June 23 Democratic primary in New York’s 16th Congressional District, a predominantly black and Hispanic district that includes parts of Westchester and the Bronx, where Jamaal Bowman, a middle school principal and outspoken advocate for racial justice, is running a tight race against the longtime incumbent, Representative Eliot L. Engel. In recent days, Mr. Bowman has received endorsements from several top progressives, including Representative Alexandria Ocasio-Cortez of New York, Senator Elizabeth Warren and Mr. Sanders.

In an interview, Mr. Bowman was reluctant to speak about his electoral odds. But he said he had received messages from supporters thanking him for speaking out against racism and racial inequality. “What we need now from Democrats and the country is a deep analysis of structural racism,” he said.

It is a sentiment that has taken on new urgency among many progressives, who stress their longstanding support for racial justice but who have also come to realize that they must more directly confront racism and police brutality than they have before.

To support the protest movement, Mr. Sanders has endorsed a slate of progressive candidates who are fighting explicitly for racial justice. Using his email list, he has raised more than $2 million for racial justice organizations, according to a spokesman.

“It’s not good enough to sit back,” Mr. Sanders said.

Although he said he thought there would be “a broadening of discussion about what we mean by justice in America,” he also held firm to his political philosophy.

“If you’re serious about racial justice — if you’re serious about criminal justice,” he said, “you have got to be serious about economic justice.”

Giovanni Russonello contributed reporting. Kitty Bennett contributed research.