Sen. Bernie Sanders of Vermont stumps for congressional candidate Eric Kingson in Syracuse, N.Y., on Friday. (Michael Greenlar/AP)

The last time Bernie Sanders came to this city, in April, he was flat-out running for president. Six thousand people packed the downtown ­Oncenter to cheer him on, and a thousand more cheered from outside.

The crowd was smaller Friday, just 500 people — but technically, they were not there for Sanders. The senator was rallying for a 70-year-old academic named Eric Kingson, who is running for Congress and who worked the crowd, praising “Bernie’s genuineness and indisputable integrity” as he promised to defend and expand Social Security.

The campaign stop, Sanders’s first for a congressional candidate, offered a glimpse of the post-presidential-bid figure he would like to become: a politician who uses his unexpectedly strong showing in the presidential race to push his progressive policies in Congress, in campaigns and across the country.

But as he ponders his next moves, and the fate of perhaps the biggest donor list in politics, Sanders is facing a challenge almost as steep as a presidential campaign. How does a revolutionary persuade his supporters to continue the revolution with someone else? Can he maintain the enthusiasm of followers who were new to politics after falling in the primaries to establishment stalwart Hillary Clinton? Can he transfer his popularity to relatively unknown figures?

And has Sanders frittered away the leverage he built by remaining an active candidate when most of the party has moved on to a bruising fall battle against Donald Trump?

In an interview with C-SPAN, Democratic presidential candidate Bernie Sanders said in the strongest terms yet that he will not be the Democratic nominee for president. (C-SPAN)

On his website, Sanders has already started to make the transition from active presidential candidate to another kind of leader. “This is your movement,” it now says, showing a montage of diverse faces.

And at the urging of his wife, Jane Sanders, he has been talking to his inner circle about launching a grass-roots organization to harness the energy of his supporters. Among aides, there is chatter about who might staff such an organization, which might resemble Democracy for America, the group that former Vermont governor Howard Dean launched following his failed 2004 presidential bid.

Sanders’s profile in the Senate is expected to increase once he returns to the chamber full time, and aides say he will almost certainly seek reelection in 2018 — though it is unclear whether either will translate into more muscle on Capitol Hill. Sanders has suggested that he will try to mobilize his supporters around key issues and that he wants to lead the Health, Education, ­Labor and Pensions Committee, which has jurisdiction over many of the high-profile issues he pushed during his campaign.

Sanders’s donor list, which the campaign says includes 2.7 million contributors across the country, is another uncertain — and highly coveted — asset. Sanders proved to be a surprisingly potent fundraiser, taking in more than $229 million as of last month, the vast majority in online, low-dollar increments.

Seeking shared values

Eric Kingson is one of the early beneficiaries of Sanders’s new role. On Friday night, in New York’s 24th House district, Sanders let Kingson speak first, and then, still flanked by Secret Service agents, he spent much of his own 24-minute speech praising the candidate, who is running in Tuesday’s congressional primary. Sanders brought Kingson and his family onstage for photos that would make the front page, as intended.

“We really can win on Tuesday — seven or eight thousand votes is very achievable,” said Sanders as he closed out. “Where is Eric’s campaign manager? Where are you?”

Zach Zeliff, a bearded 23-year-old veteran of Sanders’s New Hampshire landslide, waved his hand.

“You got the names of everybody who’s here?” Sanders said. “Okay, great!”

Syracuse showed an awkward but promising transition for Sanders. A warm-up band called Sophistafunk got the crowd to dance and cheer for “President Bernie Sanders.” Many in the crowd said the senator from Vermont was the only candidate they trusted.

“It’s my last chance to introduce my son to an honest politician,” said Stacey Edwards, 31, with 13-year-old Jonathan in tow. “I’ve been a fan of Bernie for 20 years, and I’m so glad he’s back.”

Edwards and others said they could not support Clinton for president. “I’ll go Green or I’ll write in Bernie,” she said. Then she added a caveat: “Down the ballot, I’ll vote for anyone who shares his values.”

That was what Sanders wanted to hear. Since June 16, when he asked supporters to run for office, at least 21,000 said they would — or at least hit the streets to help.

What is unclear is whether Sanders, who enjoys higher favorable numbers than Clinton or President Obama, will ever tell those voters to support the winner of the presidential primary contests.

Clinton aides have privately expressed frustration over Sanders’s continuing campaign and refusal to this point to endorse her, a step they believe could help unify the party heading into the fall.

“The intensity of his supporters is so much greater than the intensity for any other candidate, including Hillary Clinton,” said Rep. Beto O’Rourke (D-Tex.), a Clinton endorser whose El Paso-based district went strongly for her but who suggested that Sanders could pull out voters who might normally skip elections. “In El Paso, she crushed him, and yet in terms of incoming calls and emails to our office, asking where to vote, it was like 3 to 1 Bernie. In Texas, he could get some of those folks who don’t traditionally vote to come out.”

Sanders’s role is under discussion, but he has said a more immediate priority is trying to find common ground on the issues he championed during the primaries.

“It’s not just Bernie Sanders saying, ‘Oh, yes, just vote for Hillary Clinton,’ ” Sanders told CNN on Sunday. “It is Hillary Clinton standing up and saying, ‘You know what? These are the things we need to do.’ And if she does the right thing, I am absolutely confident that the vast majority of my supporters will vote for her.”

Sanders has been lobbying Clinton to embrace several of his proposals, including tuition-free public universities and a $15 minimum wage. Asked why he had yet to endorse Clinton during an appearance Friday on “CBS This Morning,” Sanders said he had not “heard her say the things that need to be said.”

That struggle is visible in Sanders’s attempts to influence the Democratic Party platform, as well. A chief reason he has given for remaining an active candidate, the negotiations illustrate both the opportunity and the limitations of his newfound status.

A draft document approved Saturday would move the party to the left on wages, banking reform and climate change, and represents several concessions by Clinton. But Clinton allies on the panel also resisted Sanders’s aggressive overtures on trade, several environmental issues and universal health care, a core of Sanders’s mission to tackle income inequality.

Is the party already united?

There is some evidence that most Sanders supporters have already fallen in line behind Clinton. In a Washington Post/ABC News poll released Sunday, just 8 percent of Democrats said they would vote for Trump; just 1 percent of Democrats and 3 percent of self-identified “liberals” said they would vote for the Green Party.

The Clinton camp, meanwhile, is moving forward with the use of other high-powered campaign surrogates. On Monday, Sen. Elizabeth Warren (D-Mass.), another darling of the left and a potential running mate, will campaign with Clinton in the battleground state of Ohio. Obama is expected to join Clinton on the campaign trail soon.

And Sanders is lending his support to candidates whose chances are just as slim as his own were. In the primary on Tuesday, Kingson faces Colleen Deacon, a 38-year-old veteran staffer for Sen. Kirsten Gillibrand (D-N.Y.), backed by both the senator and the Democratic Congressional Campaign Committee.

As of the last fundraising report, Deacon had raised $293,565 to Kingson’s $199,445. She had gone on the airwaves with slick positive ads, telling the story of a single mom who had to go on food stamps but went on to build a career for herself.

And then Sanders showed up. On Friday afternoon, at the final candidate debate before the primary, journalists at Syracuse’s NewsChannel 9 started their questions with one about Sanders and party loyalty. Over an hour, Deacon and Kingson found little to disagree about. When they did, it was on the issues that defined the Clinton-Sanders primary — whether to raise the minimum wage to a “living wage” or specifically to $15, whether to expand Social Security or merely raise the cap on the Federal Insurance Contributions Act, or FICA.

In an interview, Deacon had nothing bad to say about Sanders. “By coming to town, he’s reminding them that there’s an election on Tuesday,” she said in her downtown campaign office, as interns in the next room worked phones.

But when asked how she would define the “establishment” — the term used by Kingson — Deacon fought back. She had campaigned for paid family leave, and she had just paid off her student loans. What about that was establishment?

“Somebody might say ‘establishment’ — I say experience, I say track record, I say working with people to achieve change,” she said. “Kirsten Gillibrand and Chuck Schumer are wonderful senators for New York, and I’m proud to have their support. I don’t think that should penalized. I think it should be rewarded if you get things done.”

Early Saturday morning, Kingson’s campaign office — formerly a Sanders outpost and still festooned with Bernie pop art — bustled with volunteers. Zeliff gave quick instructions to canvassers as young at 17, as old as 80, before they headed out.

Zeliff and 24-year-old Tom ­Giancola, a fellow New Hampshire Sanders veteran, explained how canvassers would rank the voters they met from 1 (“Oh, I’m definitely voting for Eric”) to 6 (“They’re probably voting for Trump”). Then they headed out, ready to tell their neighbors that Kingson was a Bernie-supported champion for Social Security.

“Empowering volunteers to be leaders and make the difference is kind of the whole MO of the Bernie field campaign, I think,” Zeliff said. “That’s what we learned this year, and we’re not stopping.”

Wagner reported from Washington. Anne Gearan in Washington contributed to this report.