Some top Democrats won’t stop picking at the scabs of 2016, making others in the party nervous about an intra-party bloodletting that could hurt their chances of defeating President Trump next year.

Bernie Sanders’ first weeks as a 2020 candidate have been marked by a series of flare-ups between his supporters and Hillary Clinton’s old network. Bernie Bros and #StillWithHer partisans snipe back and forth online. Former Clinton aides have fueled that fight, pushing negative stories about Sanders to reporters. The betrayal and distrust on both sides continues to cut deep, and the tension only seems to be growing.

All this infighting is generating worry for Democrats from across the ideological spectrum whose top goal is to defeat Trump.

Both Clinton and Sanders allies say unity is what they want — but they’re quick to blame the other side for undercutting those efforts.

“These Bernie supporters are basically acting like it’s spring of 2016. And Sanders has often said that he has the ability to organize and move these people, yet he doesn’t stand up and tell them to cut the shit,” former Clinton aide Adam Parkhomenko told TPM. “Everyone’s saying ‘let’s stick together,’ and they are literally some of the most vicious and nasty people online.”

Sanders himself fueled the flames during a Friday interview on The View, responding with a terse “I think not” when asked if he’d reach out to Clinton for campaign advice.

“Hillary and I have fundamental — you know, fundamental differences. And that’s what it is,” he said.

David Brock, a longtime top Clinton ally, reached out unprompted to Talking Points Memo late last week to tear into Sanders — one of a handful of Clinton supporters who have done so since the Vermont senator’s announcement.

Brock blasted Sanders for a list of transgressions familiar to anyone who was around for 2016. He slammed Sanders for painting Clinton as a corrupt corporate shill and for refusing to drop out when it became clear that Clinton couldn’t be stopped from the nomination. He accused Sanders of refusing to rein in the hostile “Bernie Bro” supporters who harassed Clinton supporters online, sometimes with sexist taunts. He argued Sanders did little to help Clinton in the general election. Brock predicted all that would hurt Sanders as a candidate and potential nominee, and fretted that if Sanders doesn’t win the nomination, he could damage whoever does.

“Those things still rankle on the Clinton side,” he said. “The candidate who’s going to be strongest in unity is not Sanders… There are too many dyed in the wool Democrats who aren’t going to support him in a general election.”

Sanders’ allies fired back.

“We appreciate David Brock’s electoral advice, but Bernie’s campaign is focused on a 50-state strategy to unite Americans and defeat the most dangerous president of our lifetime,” Sanders campaign spokesperson Sarah Ford told TPM via email.

The frustration reaches to the highest ranks of both campaigns, though Clinton’s former aides have been much more vocal about their anger. Clinton allies were irritated that Sanders made former Ohio State Sen. Nina Turner a campaign co-chair, since she pointedly refused to endorse Clinton during the 2016 general election. And Clinton and her allies have been dismayed by some early attacks from Sanders’ army of supporters against 2020 rivals including Sen. Cory Booker (D-NJ), former Rep. Beto O’Rourke (D-TX) and Sen. Kamala Harris (D-CA).

John Podesta, Clinton’s 2016 campaign chairman, said he had a “respectful” personal relationship with Sanders and his wife going back decades, but said there were “scars” from 2016. He predicted Democrats would rally around Sanders if he won the nomination to stop Trump — but warned that it was on Sanders to avoid past mistakes to foster unity.

“Can they tamp down the ‘Bernie Bro’ thing that’s real and out there? People felt flamed on social media and elsewhere,” he said. “It’s kind of on them to build a campaign that doesn’t repeat those mistakes. From Hillary supporters’ perspective, I don’t think there’s a stronger group of people in America who want to see Donald Trump out of office.”

Others have been much more public with their disdain for Sanders. Former Clinton rapid response director Zac Petkanas penned an op-ed for NBC News laying out Sanders’ liberal apostasies. Jess McIntosh, a progressive analyst and 2016 Clinton aide, balked at a Sanders comment that “we have got to look at candidates, you know, not by the color of their skin, not by their sexual orientation or their gender and not by their age.”

If Bernie is going to start this contest telling us he’s at a disadvantage as a white man it is going to be a LONG year. — jess mcintosh (@jess_mc) February 19, 2019

Nick Carter, a top Sanders aide who went to work for Clinton’s campaign after the primary, said most Democrats and operatives from both sides of the divide want to move on — and make sure they unify to defeat Trump. But he expressed some concern about the burbling feud.

“The stakes are simply too high to let the pettiness from 2016 bog down what’s going to be one of the most significant voter mobilization efforts the country’s ever seen to stop Donald Trump,” he said. “Rather than fueling a regressive conversation that could leave more than a few people at home, let’s be sure we’re putting energy into what we need to do to win in November 2020.”

Sanders has made some visible efforts to tamp down this feud and avoid future nastiness. His campaign sent surrogates a memorandum condemning “bullying and harassment of any kind” and calling on them to “engage respectfully with our Democratic opponents.” He’s also promised to endorse the eventual Democratic nominee.

Sanders also made sure to praise Clinton for her civil rights support onstage at a breakfast during celebrations in Selma, Ala. honoring the 54th anniversary of “Bloody Sunday,” a seminal moment in the civil rights movement.

But the former rivals’ onstage interaction showed exactly how tense things remain between the two. Clinton gave big hugs to Sens. Cory Booker (D-NJ) and Sherrod Brown (D-OH). For Sanders, a terse handshake.