PORTSMOUTH, N.H. – Three years after Democrats witnessed a vicious presidential primary battle between eventual nominee Hillary Clinton and one-time longshot Bernie Sanders, there are rising concerns that a revival of the war of words between their two camps could hurt the party in 2020.

“I think there’s a small segment of people in both camps who harbor significant resentment,” a veteran New Hampshire-based Democratic campaign strategist told Fox News.

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“There are Bernie people who have constantly attacked Clinton and there are Clinton people who resent the primary of 2016. I think this is a lingering problem that’s going to find its way into the 2020 nomination process,” warned the strategist, who asked for anonymity to speak more freely.

Kathy Sullivan, a Democratic National Committee member from New Hampshire, urged that “it’s time for people to move on. The 2016 primary was three years ago…. We have another election coming up and that’s what we should be focusing on.”

And former state Sen. Burt Cohen pleaded: “They need to stop that.”

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“We have to put that behind us and unite,” emphasized Cohen, a leading Granite State Sanders supporter from the 2016 campaign.

Cohen was in the crowd at a joint rally at Portsmouth High School in July of 2016, when Sanders and Clinton tried to bury the hatchet in hopes of putting to rest a bitter and contentious battle for the Democratic presidential nomination that saw Sanders blast the party’s establishment favorite.

Sanders vowed “to do everything I can to ensure that she will be the next president” and Clinton thanked her rival, proclaiming she was “proud to be fighting alongside you.”

But the fragile peace between the independent senator from Vermont and the former U.S. secretary of state was tepid at best. And following Clinton’s shocking loss to GOP nominee Donald Trump in the 2016 general election, Clinton and many of her top staffers blamed Sanders and his legions for her defeat.

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Fast forward nearly three years and the bad blood is simmering anew.

Sanders, who last month launched his second straight White House bid, is the early front-runner in an ever-expanding field of Democratic presidential contenders. He tops many of the latest public opinion polls, hauled in an eye-popping $6 million in fundraising in his first 24 hours as a candidate, and drew some 25,000 people to his first two rallies.

Clinton’s supposedly out, saying in an interview on Monday that "I'm not running, but I'm going to keep on working and speaking and standing up for what I believe.”

She reportedly is keeping the door open a crack, but her Shermanesque comments certainly calmed speculation that she might seek a rematch with Trump in 2020.

"I want to be sure that people understand I'm going to keep speaking out," she added.

But it’s not what Clinton’s saying but rather comments from some of her top aides from the 2016 campaign that are roiling the 2020 waters.

Cable TV and Twitter have been the platforms in recent weeks for on-the-record and on-background soundbites and quotes from former Clinton aides and top supporters taking aim at Sanders.

Ex-Clinton staffers savaged Sanders over his use of private jets during the 2016 general election while he was stumping across the country for the Democratic nominee, as detailed in a recent Politico article.

Explanations from the Sanders camp that the senator needed to fly private jets in order to keep a non-stop itinerary of nearly 40 rallies in 13 states during the closing weeks of the 2016 campaign didn’t fly with many Clinton supporters.

Zac Petkanas, the Clinton’s 2016 campaign’s director of rapid response, fired away, saying “Royal Majesty King Bernie Sanders would only deign to leave his plush D.C. office or his brand new second home on the lake if he was flown around on a cushy private jet like a billionaire master of the universe.”

Michael Briggs, spokesman for the Sanders 2016 campaign, said in response that members of Clinton’s team are some of the “biggest a--holes in American politics.”

“You can see why she’s (Clinton) one of the most disliked politicians in America," he added.

When Sanders said in a recent radio interview that “we have go 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,” longtime Clinton policy adviser Neera Tanden fired back.

“At a time where folks feel under attack because of who they are, saying race or gender or sexual orientation or identity doesn’t matter is not off, it’s simply wrong," she wrote on Twitter.

Many in Clinton world remember the nasty online comments from some Sanders supporters during the 2016 intra-party primary slugfest. Last month, Sanders urged leading backers to “do our very best to engage respectfully with our Democratic opponents” rather than spotlight “personalities or past grievances.”

Sanders himself invited criticism from Clinton backers during an interview last week on “The View.” Asked if he would be asking for advice from the 2016 nominee – as some rivals for the nomination have been doing – Sanders answered, “I suspect not….Hillary and I have fundamental differences.”

Longtime Clinton spokesman Nick Merrill took to Twitter to punch back.

“I don’t know who our nominee is going to be but I am damn sure that beating Trump & getting America back on the right footing is going to require a unified Democratic Party, so crap like this 613 days before Election Day is irresponsible, counter-productive, & sets us all back,” he tweeted.

The war of words is worrying some longtime Sanders supporters in New Hampshire, where the senator’s crushing victory over Clinton in the February 2016 primary launched him into the bloody battle with the eventual nominee.

“They need to learn the lessons of why they lost what should have been an easy victory and just live with it so we can win [in 2020],” said Cohen, a member of the Sanders steering committee in the Granite State.

“We have to keep our eyes on the prize, which is saving America from Trumpism,” he added. “Carrying forth 2016 bitterness does no good.”

New Hampshire executive councilor Andru Volinsky called the attacks “white noise and a distraction.”

Volinsky, who served as legal counsel for the Sanders campaign in New Hampshire and helped lead the efforts to bridge the differences between the Sanders and Clinton camps in the crucial general election battleground state during the summer and autumn of 2016, argued that “I think Bernie’s done what he’s supposed to do and I think we need to focus on getting Trump out of office.”

Sullivan, a former longtime state party chair before serving as a DNC committee member, backed Clinton in the 2016 first-in-the-nation primary. But she agreed that it’s time to move past the war of words.

“I just think there’s some unnecessary sniping going on right now and that it would be in the best interest of everyone if both sides just said ‘that’s enough. Let’s just move on to the future,’” she urged.