Behind the growing fear among many Democrats that Sen. Bernie Sanders’s continued presence in the presidential race could spell doom in November is the belief that they’ve seen it happen before — in the last campaign.

The 2016 Democratic National Convention was just about to get started when Sanders (I-Vt.) addressed his delegates. It was time to support her, he told his backers. They disagreed, booing loudly. Some stuck their thumbs down as TV cameras captured the extraordinary show of dissent, which would continue on the convention floor.

To some Democrats in that campaign, it was a lesson learned the hard way about the limitations of Sanders’s promises of support and the ferocity of his unbridled backers. Four years later, with the senator still running against former vice president Joe Biden despite almost impossible odds of victory, some party leaders are increasingly worried about a reprise of the bitter divisions that many Democrats blame for Hillary Clinton’s loss.

“It’s the equivalent of a World War II kamikaze pilot,” said Philippe Reines, a longtime adviser to Clinton. “They have no better option than to plow into USS Biden.”

The judgment Sanders makes about his fate and the direction taken by his supporters could be among the most consequential decisions of the race, determining whether Democrats speak with one voice against a president who is already aimed at November or squabble for months more.

Although Sanders has long pledged to do all he can to help the eventual nominee defeat President Trump, Democrats are still haunted by the last grueling battle, which didn’t end after it became clear that Clinton would be the nominee, and instead stretched into the summer convention and beyond. Then, as now, an impassioned band of Sanders supporters voiced their displeasure loudly and widely, sometimes echoing the harshest attacks of Trump and his allies with little reproach from Sanders.

Moved by an urgency to come together against Trump as the coronavirus pandemic has upended the presidential race, some party leaders feel that Sanders should end his campaign and help the Democratic Party position itself for the November general election.

“I just think it’s a terrible decision for him to make because he looks very selfish,” said former Democratic senator Barbara Boxer of California, who backs Biden. If Sanders is genuine about going all in to defeat Trump, “then get out,” she said.

But Sanders has given no indication that he is preparing to do that. He recently said he wants to debate Biden in April. His team announced it is expanding digital organizing efforts ahead of the New York primary, which on Saturday was moved from April 28 to June 23. And Sanders has signaled a strong desire to use his campaign megaphone to advocate for liberal policies like Medicare-for-all — which allies said are more crucial than ever because of the public health crisis.

Some people close to Sanders voiced confidence that the senator would stay in the race until the July convention, though they said they had received no final word on his plans. Others close to the senator have sounded less certain of that, noting only that he continues to be a candidate at the moment.

One of the people close to Sanders — who, like the others, spoke on the condition of anonymity to reflect internal deliberations — said Biden would have to make significant policy and personal overtures to Sanders to potentially persuade the senator to leave the race, and to win the trust of his followers.

On Wednesday, Sanders delivered a fiery address on the Senate floor excoriating his Republican colleagues for trying to reduce the financial support some workers could get in a coronavirus bill. “Oh my word, will the universe survive?” a gesticulating Sanders wondered aloud, suggesting sarcastically that giving struggling workers a bit more would be devastating.

For many loyal supporters, the address was a stark reminder of why they feel it’s important for him to stay in the contest. “This is why @BernieSanders should be President of the United States,” the activists known collectively as “People for Bernie” tweeted to 228,000 followers.

Some Democrats, including veterans of the 2016 contest, said they see signs of hope that the party can avoid the intraparty viciousness that marked that race. Sanders has a much better relationship with Biden than he did with Clinton, they noted, and for the moment he has ceased direct attacks on the former vice president. Plus, they added, the Clinton-Sanders fight was a bout of more than a year and left deep battle scars.

“Last time, it was a two-way race — it was person on person. And that just made for more conflict. Here, there was a lot of conflict, but it flew in 15 different directions. Among the final run of candidates, it went every which way,” said Mark Longabaugh, a top Sanders strategist in 2016 who worked in 2020 for Andrew Yang, one of the many rivals Biden and Sanders fended off.

Sanders argued in a Thursday radio interview with “1A,” a program broadcast on NPR, that the postponement of upcoming primaries because of the pandemic makes this race “very different than 2016.” He ticked through steps his campaign has taken to adapt to a landscape he said had “changed very profoundly.”

Although Sanders himself is not waging a scorched-earth campaign against Biden, some of his most visible supporters continue to rail against the former vice president’s policy ideas and question his cognitive abilities — a trend that worries party leaders.

“Biden begins to ramble an incoherent point about being proud of some people, but stops himself mid-sentence on air and just ends the thought,” tweeted Shaun King, an activist who supports Sanders and has introduced him at events. He was commenting Tuesday on the TV interviews Biden had done about the coronavirus. Trump allies have launched similar strikes on Biden.

Senior Democrats have expressed concern in recent days that Sanders is once again obliquely giving his supporters permission to continue to question Biden’s fitness as the Democratic nominee.

“There is growing anticipation for him to start to help,” said one senior Democratic strategist, who spoke on the condition of anonymity to speak more frankly about the concerns. “For his movement to be successful, he needs to find the right way to land the plane at Joe Biden international airport.”

Inside the Biden campaign, apprehension peaked during the last debate, when Sanders went beyond the policy discussion that he said he was seeking and moved instead to attack Biden’s honesty and question whether his current promise to protect Social Security could be trusted. After the debate, Anita Dunn, a senior Biden adviser, punched back by comparing Sanders to “the kind of protester who often shows up at campaign events on live television.”

For some Clinton alumni, the debate brought back bad memories of the spring contests last cycle. In April 2016, the Sanders campaign was pushing videos of Clinton talking about her “wonderful donors” and campaign ads that talked about Washington politicians who are paid “over $200,000 an hour for speeches” from Wall Street firms. Those messages became central themes in Trump’s campaign against Clinton, who he called “corrupt” all the way through the general election.

Clinton defeated Sanders in New York in mid-April, a defeat that made it nearly mathematically impossible for Sanders to overtake her lead in delegates. But he continued his campaign. Clinton in June won the California primary, closing off Sanders’s path to the nomination for good, yet he did not endorse her for another five weeks.

Shaping the party platform that would be ratified at the convention was one Sanders’s main goals in keeping the campaign alive, Longabaugh recalled.

“He talked about wanting to move the party and move it to the left. That was one of the objectives of his presidential candidacy, in addition to trying to win. I think the platform became his mechanism for doing that,” Longabaugh said. “He also had millions of supporters who had contributed to his campaign and traveled to other states to campaign. He felt they deserved a chance to cast a ballot for him.”

Many Sanders supporters argued that they made real efforts to work together with the Clinton team at the convention. One indicator of high-level cooperation: The Sanders officials and Clinton’s aides worked out of the same workspace. Sanders also made another key concession to avoid discord, according to Longabaugh: not escalating a fight over trade, Medicare-for-all and convention “superdelegates” during negotiations among party insiders.

Nomiki Konst, a liberal strategist, was among the Sanders delegates placed on the platform committee. Over two hot days in an Orlando hotel ballroom, their mission was to get as much of the Sanders agenda as possible into the platform. They managed to influence the document with ideas such as a $15 dollar minimum wage.

“When the California primary was over, we knew he’d lost the nomination, but getting more delegates meant that we had more members of the platform committee,” Konst said.

Some Sanders allies say their goal is similar this time, and they are publicly urging Sanders to stay in the race to gain leverage over the proceedings.

“A political party is supposed to be a place where you actually debate. There are huge differences here,” said Larry Cohen, a Sanders friend who heads a nonprofit aligned with the senator. “There needs to continue to be a reform movement in this party, not a coronation.”

Supporters of both Clinton and now Biden, however, say they dramatically moved in Sanders’s direction during the campaigns. Clinton adopted liberal policies in 2016 to reach out to Sanders’s backers; Biden more recently has adopted programs that Sen. Elizabeth Warren (D-Mass.) and others have proposed, and he has rhetorically invited Sanders’s supporters into his campaign.

Among the moderates, there remains a frustration that the Sanders forces demand that the winning primary candidates conform to his views, and not the other way around. They suggest the situation is even more dire this year than in 2016, given the party’s antipathy toward Trump.

Biden, for his part, has sought to strike a welcoming posture toward his former Senate colleague. Both teams remain in touch on the coronavirus crisis and its effect on the economy, according to one Biden adviser.

It is their personal dynamic — they are two men of similar ages who both served in Congress and who associates said genuinely hold each other in high regard — that gives some Democrats hope for averting the ugliness of 2016.

“I sense that there is a personal respect that he has for Joe Biden that he did not extend to Hillary in 2016, so I assume that that will extend some guard rails,” said Brian Fallon, who worked as press secretary for Clinton’s campaign. The bigger challenge, Fallon said, will be reining in Sanders supporters while their candidate extends his presence in the race.

Scott Brennan, a Democratic National Committee member from Iowa, expressed hope that the party’s divisions will not repeat because Biden “doesn’t generate the same sort of fevered hatred” from Sanders allies that Clinton did.

But other veterans of the last campaign expect the worst.

“Everyone should stop pretending that Bernie is doing anything other than helping Bernie,” Reines said.