Both in tactics and rhetoric, there are growing signs he takes his rival very seriously — and that he increasingly views Sanders as his most formidable opponent in Iowa and beyond.

The Biden campaign has specifically courted the endorsement of community leaders in Iowa who backed Sanders in 2016. They’ve sought to combat Sanders’ recent habit of rolling out star surrogates like Alexandria Ocasio-Cortez with their own A-list surrogates. And last week, Biden’s five-day Iowa bus tour heavily concentrated on the eastern part of the state — the biggest regional battleground between the two candidates because of its concentration of working-class voters.

“They have to start forcing Bernie to address some of his obvious challenges,” said Simon Rosenberg, founder of the New Democrat Network and a senior strategist for the Democratic Congressional Campaign Committee in 2018. “The gloves-off strategy didn’t work for Clinton, and it isn’t going to work this time either.”

In 2016, Hillary Clinton was criticized for underestimating Sanders — something she denied at the time – and assuming she’d steamroll her way to the nomination. Instead, she endured a protracted primary that left Sanders supporters embittered and unenthusiastic in the general election. Clinton barely edged out Sanders here by three-tenths of a percentage point, 49.9 percent to 49.6 percent, in a contentious election that left long-lasting scars.

While a crowded primary field means the dynamics are considerably different this year, Biden campaign advisers still acknowledge Sanders presents a formidable challenge. He began the 2020 contest in Iowa with a deep and loyal following — and a base that’s clamoring for a rematch with an establishment candidate. Biden aides say they respect the level of excitement and loyalty from Sanders backers and recognize the threat that the Vermont senator’s fundraising juggernaut presents.

“What is the lesson from 2016? It’s to not underestimate Bernie Sanders and his supporters,” said Biden senior adviser Anita Dunn. “It’s a strong, broad base of support. And he is an indefatigable candidate.”

While the two candidates represent different wings of the party, polling data suggests another reason for concern: Sanders, more than any other top tier candidate, comes closest to rivaling Biden’s appeal to minority voters and working class voters.

“I never would underestimate Bernie,” said Randy Black, Cerro Gordo County Democratic Party vice chair, who has not endorsed a candidate yet. “He’s more than ever a threat.”

Since the start of the year, Biden has sharpened his messaging to highlight the distinctions, dedicating a bigger portion of his stump speech toward making the case that he is the candidate best-suited to work with Republicans and heal a divided country — something the campaign says draws a sharp contrast with both Sanders and Elizabeth Warren.

As Biden hits his stride in Iowa, that theme represents a big part of his closing argument. As part of the case for his electability, he asserts that he, more than any other candidate, can help win back the Senate by providing a top of the ticket boost in places like North Carolina, Arizona and Texas — states where there are questions about whether a candidate as liberal as Sanders would be an asset to the eventual Democratic Senate nominee.

“We think it is a clear point of difference with the approach of some of the other major candidates and one that is authentic to Biden and his record,” Dunn said.

Biden has recently sought to counter Sanders’ practice of rolling out star surrogates like Ocasio-Cortez, who drew massive crowds in the state in November and electrified Sanders’ supporters. Her tour through Iowa drew more than 2,000 people at each of the events.

Biden’s relatively staid endorsement events here couldn’t be more different from the razzle dazzle rallies showcasing Ocasio-Cortez. But they’ve featured political heavyweights designed to highlight Biden’s messaging about his experience and electability.

Former Secretary of State John Kerry — who won the Iowa caucuses en route to the 2004 Democratic nomination — was the headliner among a group of Democratic officeholders who embarked on a "We Know Joe" bus tour this week.

Former Gov. and ex-Secretary of Agriculture Tom Vilsack and his wife Christie, recently toured with Biden through rural Iowa in small, intimate venues. Rep. Abby Finkenauer — a rising Democratic star in the state — campaigned with Biden in mid-size events, the largest bringing about 700 people.

Biden advisers and surrogates have framed the endorsements as clear markers of the differences between Sanders and Biden, with Ocasio-Cortez representing the left flank of the party and Finkenauer representing the mainstream — she knocked off a Republican incumbent in a Northeast Iowa-based swing district, the kind of place that the Democratic nominee will need to defeat Donald Trump in November.

“Ms. Ocasio-Cortez represents a district in New York, so she’s probably reflective of the values of the people in that district in New York,” said Rep. Ami Bera of California, who is among the surrogates traveling through the state on Biden’s behalf this week. “I’d say Abby Finkenauer represents a district in Iowa and probably is reflective of the values of the people in that district. If you’d ask me which endorsement I’d want, I’d want the endorsement of a Gov. Vilsack or an Abby Finkenauer, in the state that I’m running in.”

To that end, Biden snagged several endorsements that went to Sanders in 2016, including Waterloo pastor and African American leader Frantz Whitfield, former AARP Iowa director Bruce Koeppl, Sioux City state Rep. Tim Kacena and Linn County Supervisor Brent Oleson.

If his recently amped up criticism of Biden is any indication, Sanders likewise views the former vice president as a threat. With 26 days to go until the caucuses, the race has tightened: a recent CBS/YouGov poll showed Sanders, Biden and Pete Buttigieg in a three-way tie for first place, with Warren not far behind.

Yet there’s been little pushback to the Sanders attacks from the Biden campaign, or even his top surrogates. One reason is that, since entering the race last April, Biden has sought to position himself as the antidote to Trump and not get sucked into personal exchanges.

But advisers also say there’s little to gain by attacking Sanders, given that at least a portion of their constituencies overlap. And so far in the primary, there’s evidence that negative hits on rivals aren’t working — candidates who’ve hit Biden the hardest, for example, have gone nowhere. Biden’s team believes that unlike 2016, the 2020 primary is defined by who can beat Trump, and poll after poll shows Democrats believe Biden is best suited to do that.

“I don’t think anybody on the Biden campaign is naive about Bernie’s very real chance of winning the nomination,” said Liz Allen, a former aide to both Biden and Barack Obama, and a Biden campaign surrogate. “I just think it’s doubling down on their strategy, which is to make the case about Donald Trump.”

