It’s hard to imagine right now but Bernie Sanders and Joe Biden both know that in a few months they’re going to have to put on a united front to have any chance of beating Donald Trump.

Over the last few days, as the former vice-president has collected the majority of delegates needed to clinch the Democratic nomination for president, Sanders, the Vermont senator and liberal icon, has opted to press on in the primary. That sets up an ongoing bruising battle between the two candidates for the immediate future even as the vast majority of observers believe Sanders will eventually have to grit his teeth and fall in line behind Biden.

No major 2020 Democratic candidate, current or former, has told supporters not to back the eventual Democratic nominee. Likewise, no serious Democratic surrogate has urged supporters to sit out if their candidate doesn’t win. It is a precedent made even more urgent given Democrats’ anger at Trump and fears over him winning a second term.

But the fierce fight between the progressive and centrist wings of the party has had an impact as the party begins to look ahead to the November battle.

Saikat Chakrabarti, the former chief of staff to the New York congresswoman Alexandria Ocasio-Cortez and the president of the New Consensus network, said the worry is not getting progressives to vote for a nominee they may not have backed in the primary. Chakrabarti said it was rallying voters who stayed out of the general election in 2016.

“The thing I’m worried about is if Joe Biden wins the nomination, what will he do to get actual voters,” said Chakrabarti, “the kinds of folks who stayed home in 2016, and I mean disaffected African American voters, working class voters, people who are really having a tough time with the current economy.”

As the primary has shifted essentially to a two-man race, the rivalry between Biden and Sanders and their respective supporters has intensified. Interviews with over more than a dozen Democratic strategists and officials yields a sense of confidence that all the participants in the primary will shift their focus to defeating Donald trump in the general election.

But these Democrats also caution that to erase hard feelings left over from the primary, the Democratic nominee will have to extend some kind of olive branch to the runner up and his supporters.Biden will have to work hard to win over disappointed grassroots progressives if he wins as expected. In the now highly unlikely event Sanders wins, he will have to ease concerns from wary Democratic moderates.

“I think there has to be a set of actions,” said Steve Grossman, a former Democratic National Committee chairman. “I think there has to be a consistent expression of respect for the progressive wing of the Democratic party that is palpable and meaningful and smart and savvy and thoughtful.”

In recent days Biden and his top aides have said urged Sanders supporters to come support their campaign. Similarly on Thursday, Jeff Weaver, a senior Sanders adviser, said the Vermont senator would support Biden if the former vice-president got the nomination. Weaver said Sanders would “absolutely” bring his supporters into the fold even if he didn’t get the nomination.

“If Joe Biden is the nominee Bernie Sanders will support him wholeheartedly, we’ll campaign for him,” Weaver said.

During Biden’s election night speech on Tuesday he thanked Sanders and his supporters “for their tireless energy and their passion”.

“We share a common goal and together we’ll defeat Donald Trump,” Biden said. “We’ll defeat him together. We’re going to bring this nation together.”

But for now, with the contest still live and heading for a debate in Arizona on Sunday, Sanders and his supporters are showing no sign of falling in line behind Biden. Progressive Sanders supporters still see a narrow path to the nomination , even as Biden’s delegate lead becomes insurmountable. For them, simply defeating Trump isn’t enough.

Supporters of Bernie Sanders wave signs at a rally in Chicago on 7 March. Photograph: Charles Rex Arbogast/AP

Jennifer Epps-Addison, network president of the Center for Popular Democracy, which endorsed Sanders, said Democrats cannot rely on antipathy to Trump to turnout young people and progressive activists inspired by Sanders campaign in November.

“If they want our time, our energy and our organizing efforts, then they’re going to have to agree to actually include us in the leadership and the governance of the party,” said Epps-Addison.

She added that if Biden is the nominee it will matter who he chooses to be his running mate, warning against anyone with a problematic record on criminal justice.

“We are very concerned that Donald Trump right now is poised to run to the left of Joe Biden on criminal justice reform,” Epps-Addison said. “It is a terrifying prospect for those of us who are knocking doors in black and brown communities that have been terrorized by the crime bill that Joe Biden was the architect of.”

In a scenario where Biden wins the nomination, he will have to consider embracing some of the issues Sanders campaigned on, said Jared Leopold, a veteran Democratic operative.

“I think a lot of it is going to be about leaning into the issues that they care about,” Leopold said. “That doesn’t mean that Joe Biden needs to fundamentally change who he is, but I think talking about issues that young people care about and progressives care about will be important to mobilizing those voters in the fall.”

Mark Longabaugh, a senior adviser to Sanders’ 2016 campaign, said that “any kind of marriage is a two way street”.

“Biden, if he is the nominee, would be wise to reach out and make an effort to make Sanders and his supporters feel welcome in a Biden coalition,” Longabaugh said.

And right now, Sanders has a better chance of exerting influence over the main committees at the Democratic national convention in July than winning the nomination.

“The more delegates you have clearly the more leverage you have,” Longabaugh said. “And he has plenty of delegates.”

He added: “And my own view is that if Bernie Sanders does not have a good night next Tuesday, he needs to seriously think about getting out and rallying around the Joe Biden candidacy.”