For the last decade, Senator Bernie Sanders of Vermont has been running for president, planning a run for president or pushing former President Joseph R. Biden Jr. to adopt more progressive policies.

But now, as Democrats find their legal and fund-raising institutions under attack from the Trump administration, their base voters furious at their congressional leadership and their party’s popularity at a generational low, progressives are also staring down the prospect of a post-Bernie future.

A movement politician with a large and devoted base of supporters, the 83-year-old Mr. Sanders has signaled that he does not intend to run for president again. The question now is who will lead the network he built from scratch into the next presidential election and beyond.

Interviews with nearly 20 progressive Democrats about the left wing’s future revealed a faction that sees the ideas Mr. Sanders has championed — reducing the power of billionaires, increasing the minimum wage, focusing more on the plight of workers — as core to the next generation of mainstream Democratic politics.

Though there is little agreement about who will emerge to guide progressives into a post-Sanders era, virtually everyone interviewed said there was one clear leader for the job: Representative Alexandria Ocasio-Cortez of New York.

And it just so happened that Mr. Sanders and Ms. Ocasio-Cortez spent three days last week on a “Fighting Oligarchy” tour through Arizona, Nevada and Colorado. In Denver, they drew 34,000 people, what Sanders aides said was the largest crowd of his career. Neither has so much as obliquely referred to the torch-passing nature of their trip, and in an interview, Mr. Sanders declined to answer questions about whether Ms. Ocasio-Cortez, 35, would inherit his mantle. But the subtext of their travels appears clear.

She is what’s next — if she wants it.

“Alexandria has been doing an extraordinary job in the House,” Mr. Sanders said. “You can’t sit back. You can’t wallow in despair. You’ve got to stand up, fight back and get involved in every way that you can. There’s nobody I know who can do that better than Alexandria.”

Ocasio-Cortez’s three options

Ms. Ocasio-Cortez, who declined an interview request, has said nothing publicly about her political plans. Several people who said they had spoken with her relayed that she was far from making any decisions.

But the fourth-term congresswoman has three clear options.

She could focus on the House, where she has become a well-liked and respected member of the Democratic caucus, and try to become a committee chairwoman if Democrats win back a majority in next year’s midterm elections.

She could run for the seat now held by Senator Chuck Schumer of New York, the minority leader. Or she could seek the presidency in 2028.

(Ms. Ocasio-Cortez has also mused about dropping out of politics altogether, the people who had spoken with her said. This seems less likely, given her lengthy admonition on Thursday to a crowd in North Las Vegas, Nev., to stay involved in the fight against the Trump administration.)

Her evident frustration with Mr. Schumer after he greenlit the passage of a Republican spending bill this month heated up the long-simmering conversation about whether she might run for his seat in 2028, whether he seeks a sixth term or not.

A person who has worked with Ms. Ocasio-Cortez on campaigns, and who insisted on anonymity to discuss private outreach, recounted being inundated with calls from Democrats — and not just those on the far left — after Mr. Schumer’s vote, asking about the congresswoman’s future and encouraging her to consider higher office.

“She’s not looking to jump to the next thing or the next thing or the next thing, just for the simple reason of jumping to that thing,” said former Representative Jamaal Bowman of New York, a political ally and friend of Ms. Ocasio-Cortez’s. “When everyone is saying, ‘Speed up,’ that’s actually the time to slow down.”

He added: “You got to take a breath. That race for the U.S. Senate is three years away. Let’s govern for a little bit.”

Ms. Ocasio-Cortez has also had extensive conversations with House allies like Representative Jamie Raskin of Maryland, who said he had spoken with her about his own deliberations over whether to run for the Senate. Ahead of last year’s election, Mr. Raskin decided to remain in the House, passing on a Senate race in which he would have been a heavy favorite.

The Senate provides a larger megaphone for politicians, Mr. Raskin said, but he believes they can accomplish more of their policy goals in the House, a prospect that may appeal to Ms. Ocasio-Cortez — who hardly needs a larger megaphone.

“We are in a moment of crisis, and a crisis is always a moment when new leadership surfaces to speak to the moral and political imperatives of the time,” Mr. Raskin said. “This crisis may be the end of some people’s political careers, and it may be the beginning of some people’s political careers.”

The question of who could assume the Sanders mantle — at least in part — is all but certain to come to a head in the next presidential election.

In some ways, the jockeying is already evident.

“I don’t think there’s going to be, in my view, a standard-bearer or two standard-bearers or three standard-bearers for the progressive movement,” said Representative Ro Khanna, a California Democrat who has had conversations to game out a potential 2028 presidential campaign of his own. “We’re going to see the beginning of a new progressive era where we’re going to see successive progressive nominees.”

Of course, there are deep divides in the party over how far to the left Democrats should go.

But some also argue that the tensions in today’s Democratic Party no longer center on the kinds of ideological clashes that characterized the 2020 primary race — left versus moderate and litmus tests on issues like single-payer health care.

At least for now, these Democrats say, the debates concern how and where to draw the line against President Trump and Elon Musk, the richest man in the world.

Several mainstream Democrats, including Senator Chris Murphy of Connecticut, Gov. Tim Walz of Minnesota and Gov. JB Pritzker of Illinois, himself a billionaire, have drawn attention from progressive activists because of their vigorous pushback to the Trump administration.

“The biggest split amongst Democrats is between those who want to stand and fight and those that want to play dead,” said Representative Greg Casar, a Texas Democrat and the chairman of the Congressional Progressive Caucus. “We need more leaders from the stand-and-fight wing of the Democratic Party.”

Danielle Brecker, a leader of Empire State Indivisible, which has called on Mr. Schumer to step aside as minority leader, said she saw Ms. Ocasio-Cortez as the “future of the party” with any number of promising paths.

But she questioned the country’s willingness to elect a woman in 2028.

“I sadly think that it probably needs to be some very safe white man,” she said. “I feel terrible saying that. That wound is still very sore.”

When Mr. Trump was in office the first time, the liberal energy was firmly with Mr. Sanders.

By the time the 2020 Democratic presidential primary race began, several contenders rushed to embrace Mr. Sanders’s goals on heath care and other issues — even though most of the party’s successful candidates in the 2018 midterm elections adopted more moderate stances. The party’s eventual nominee in 2020, Mr. Biden, took a more moderate tack as well.

There are now signs that outraged constituents have regained the power to steer their elected officials. Anger at Mr. Schumer and Senate Democrats last month prompted futile displays of opposition to Mr. Trump’s cabinet appointments. Democrats hosting town-hall events meant to hold Republicans accountable for the Trump agenda have found themselves facing liberal pushback for not being able to change the country’s course.

Taking the nation in a new direction will require, some progressives said, a sustained effort to demonstrate both popular opposition to Mr. Trump’s agenda and support for a liberal alternative.

“After the murder of George Floyd, you saw a massive outpouring,” said Keith Ellison, the Democratic attorney general of Minnesota, whom Mr. Sanders backed in 2017 to become the Democratic National Committee chairman. “You saw a lot of people making statements about police accountability, diversity, equity and inclusion. And then when the movement subsides, they’re ready to roll it all back.

“So there’s a lesson there. The lesson is you’re going to have to stay in the streets.”