Hard feelings linger in campaign politics.

“After every primary cycle, there’s always bad blood because people spill a lot of blood in the process,” said former Democratic National Committee chairwoman Donna Brazile, a Fox News contributor. “Unfortunately political wounds don’t heal as quickly as physical wounds.”

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Need proof? Look no further than the 2020 Democratic presidential fight.

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

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

The strategist, who asked for anonymity to speak more freely, warned that 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.”

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Sanders and Clinton tried to bury the hatchet in the summer of 2016, in hopes of putting to rest a bitter and contentious primary fight for the Democratic presidential nomination that saw Sanders blast the party’s establishment favorite.

But the delicate 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.

Fast forward to 2019 and the ill will lingers.

Some of Clinton’s top aides from her 2016 campaign took aim at Sanders earlier this year, as he came out of the gate on a roll following the February launch of his second straight presidential campaign.

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 Politico article.

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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 some of those Clinton aides.

“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,” Zac Petkanas, the Clinton’s 2016 campaign’s director of rapid response, said.

The spokesman for the Sanders 2016 campaign fired back, claiming 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," Michael Briggs added, speaking to Politico.

Sanders did himself no favors in an appearance earlier this year 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.

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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 rocketed 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],” urged former state Sen. Burt 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.”

The call for unity to oust the Republican president from the White House in the 2020 election was echoed by Kathy Sullivan, a former longtime New Hampshire Democratic Party chair who for the last decade served as a Democratic National Committee member.

“I think that everyone wants to beat Donald Trump and that’s the most important thing. I think most people would say ‘that’s over and done with and let’s focus on 2020.’ Let’s not hurt ourselves,” noted Sullivan, who backed Clinton in the 2016 primary.

Judy Reardon, a veteran Granite State-based Democratic strategist who also supported Clinton last time around, said she’ll back whomever wins the nomination.

“People like me who supported Hillary Clinton are very practical and will support the Democratic nominee for president. To the extent there are hard feelings, I don’t think they’ll impact how people vote.”