MANCHESTER, N.H. — Virtually every poll since 2016 has shown Bernie Sanders would beat Donald Trump.

But Sanders’ aides know that some Democrats don’t buy it: Doubters just can’t picture the people of, say, rural Pennsylvania putting a 78-year-old democratic socialist in the White House. It strains their very idea of America itself.

“We still have to work to do,” Faiz Shakir, Sanders’ campaign manager, told POLITICO. “Amongst the Democratic primary electorate, in some pockets, there’s fear of how he’s going to campaign on Medicare for All: ‘Are people going to get nervous and run away from it?’”

Sanders’ aides have long believed that persuading voters he can oust Trump is key to winning the primary. It’s been a central part of their strategy since the earliest days of their 2020 campaign, and they’ve crafted a campaign tour, speeches and digital P.R. push aimed at winning the argument. But after months of work, they’re still running up against voters who support Sanders’ bid but just can’t see it happening, no matter what the poll numbers say.

"I love Bernie,” said Gloria Hoag, a New Hampshire delegate, at the state Democratic Party’s annual convention last weekend. “But I don't know if he can beat Trump because he's so far to the left. We need someone who's a little more moderate.”

Democrats are petrified that Trump will win a second term, and many are telling pollsters that they care more about whether candidates can take him on than their policy positions. Joe Biden is the front-runner in part because so many voters are looking at the primary through the eyes of a pundit. Elizabeth Warren, who is tied with Sanders for second place according to Real Clear Politics’ polling average, has risen in the polls as she’s persuaded larger numbers of Democrats she can win.

But Sanders hasn't had as much success convincing more voters he has what it takes to defeat Trump, possibly complicating his path to the nomination.

At a canvassing kickoff in New Hampshire on Saturday, Sanders’ campaign co-chair, Ben Cohen, pinpointed the problem. He wanted to be sure that the hundreds of volunteers and staffers in the crowd heard what he said before they left to knock on voters’ doors.

“For so many people out there that are going to vote in this primary, the most important issue is beating Trump,” said Cohen, the co-founder of Ben & Jerry’s. “And for most people, the general idea that's out there in the mainstream is that the person who is going to beat Trump is the centrist, Biden. But the reality is that in poll after poll after poll, Bernie beats Trump.”

Sanders hammered away at the same message minutes later: “We are defeating Trump in every national poll that has ever been done," he told his army of supporters.

Sanders has tried to persuade Democrats that he’s their best fighter largely through data, but he has attempted other approaches at times. The Vermont senator took a four-day "electability tour" in April through Pennsylvania, Wisconsin and Michigan — the Rust Belt states that were critical to Trump’s 2016 victory — as well as other parts of the Midwest to demonstrate that he can win over working-class voters.

But it's Warren who has made the biggest gain in perceived electability recently. The percentage of voters considering her who think she would probably oust Trump has gone from 39 percent to 55 percent in the last three months — a 16-point leap — according to polling by CBS/YouGov. Other surveys paint a similar trajectory.

For Sanders, the polling is more mixed. He got good news this week when CBS/YouGov found that 58 percent of voters eyeing him believe he would probably beat Trump, up from 50 percent in June. But according to polls by the Economist/YouGov, 52 percent of likely Democratic voters currently think Sanders would probably win an election against Trump — the same number as those who did so in June.

Biden, meanwhile, remains ahead of both Warren and Sanders in perceived electability in numerous polls.

Sanders is facing a challenge for multiple reasons, Democratic operatives said: He is seen by some voters as simply too liberal to beat Trump, more the leader of a robust movement than a commander-in-chief. Republicans would try to scare center-left suburban voters by painting him as a leftist radical, skeptics of the Vermont senator have said.

While Sanders leads Trump in both national and battleground state polling, Biden is further ahead — which could be important to Democrats in an election in which they see no room for error. It's also possible Sanders isn't talking about electability the right way.

“His message tends to focus on what he wants to do and where he wants to take the country on issues. It’s not so much a comparison with Trump,” said Jeff Link, a Democratic strategist in the first-in-the-nation caucus state of Iowa. “That’s probably why he hasn’t moved a lot. ... Certainly Biden talks about Trump more.”

Sanders’ aides believe that the media is also hurting his cause. Pundits portray Biden as the electable candidate, they argue, dismissing polls showing Sanders would defeat him as well.

Despite the fact that Warren has made more gains in the area recently, Sanders’ aides think he'll win the nomination because voters will come to see Sanders as the candidate best positioned to both defeat Trump and deliver change.

Medicare for All is a key part of that. Though they admit some voters are wary of a general-election backlash over it, his staffers think more and more people will eventually gravitate toward his candidacy precisely because of his far-reaching health care proposal. In their view, Trump won, in part, by parroting Sanders’ populist plans, such as cutting prescription drug prices and negotiating pro-worker trade deals.

"He campaigned as a fake Bernie Sanders," said Shakir. "The way you beat him is with a real Bernie Sanders."

It’s striking how differently the leading Democratic candidates talk about electability. Biden has put the issue at the center of his campaign, and his first TV ad in Iowa used a number-driven approach. It trumpeted that “all the polls agree: Joe Biden is the strongest Democrat to do the job.”

Instead of talking about polls, Warren has sought to upend the way voters think about electability altogether. In the second primary debate in July, she quipped, “I remember when people said Barack Obama couldn't get elected. Shoot, I remember when people said Donald Trump couldn't get elected.”

Kamala Harris has taken a similar approach, arguing that pundits dismiss African-American and female voters when considering who can win the Midwest: “It’s short-sighted. It’s wrong,” she said.

In his most successful moments, Sanders’ efforts to prove he’s electable has gone viral. During a Fox News-hosted town hall earlier this year that was held in a Pennsylvania county that backed Trump, audience members cheered when they were asked if they wanted to switch to Sanders’ Medicare for All plan. The intended message was clear: Sanders’ populism can defeat Trump in the battleground states that matter.

Other times, his efforts have fallen flat. Ahead of the first debate, Sanders’ campaign signaled that he would take on Biden over electability. But Sanders failed to aggressively confront Biden, and in the run-up to the second debate, his team settled on a different strategy: put Medicare for All center-stage.

Mark Longabaugh, a top strategist for Sanders during his 2016 presidential bid, said the conventional wisdom on electability gets it backwards: The first step is for a candidate to make the sell to voters on values and issues. Then, if a candidate succeeds on that front, the rest comes naturally. Warren, he said, is probably seen as more electable by voters these days because she's won their support — not the other way around.

"This whole electability thing is overblown," he said. “I remember the early feedback we got in 2015 in Iowa and New Hampshire: A lot of people said they loved Bernie’s message, absolutely loved it, but they just assumed Clinton was going to win. Over time, as the campaign developed and his standing progressed, they stopped saying that. Because they liked him.”