Democrats who believe Sen. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.) had a negative influence on the 2016 general election against President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE are increasingly expressing worries he’ll hurt the party again in 2020.

The Democrats complaining about Sanders, some of whom have histories with former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE’s presidential campaign, argue the rhetoric being employed by the Vermont senator in some cases goes too far in damaging his rivals.

They say it will make it harder for the party to unify around a nominee, and they’re particularly worried that supporters of Sanders won’t back any nominee who isn’t their favored candidate.

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“He needs to stop,” said one Democratic strategist, who is not affiliated with any of the presidential campaigns. “It's not helpful and it actually hurts the party. It’s like he didn't learn his lesson the last time. It’s incredibly short-sighted and terrible.”

This strategist pointed to the senator’s recent remarks in a Los Angeles Times editorial board meeting where he said that Trump would eat former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE’s lunch if he is the nominee.

“Joe Biden is a personal friend of mine, so I’m not here to, you know, to attack him, but my God, if you are, if you’re a Donald Trump and got Biden having voted for the war in Iraq, Biden having voted for these terrible, in my view, trade agreements, Biden having voted for the bankruptcy bill. Trump will eat his lunch,” Sanders told the Times.

Biden isn't the only rival Democrat taking fire from Sanders, and it's not just those who worked on the Clinton campaign who are complaining about him.

Sen. Elizabeth Warren Elizabeth WarrenWarren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon No new taxes for the ultra rich — fix bad tax policy instead MORE (D-Mass.), the progressive candidate who has been a friendly rival to Sanders, on Sunday criticized her fellow candidate. Warren said she was "disappointed" that the Sanders campaign had been using a script for its volunteers to tell potential voters that Warren was appealing mainly to upper-income Democrats and would not bring in new supporters to the party.

“I was disappointed to hear that Bernie is sending his volunteers out to trash me,” Warren said. “I hope Bernie reconsiders and turns his campaign in a different direction."

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Veterans of the Clinton campaign say it all looks similar to 2016.

“In and of itself it shouldn't take on too much meaning,” said Philippe Reines, a longtime communications adviser to Clinton.

“But if it's a harbinger of tone to come, that's not great. And I say that as someone who doesn't believe the primaries should be a cotillion or game of paddy cakes.”

Sanders allies bristle at the criticism, which they see as unfair to their candidate.

They say Sanders isn’t doing anything other candidates aren’t doing in criticizing a fellow Democrat on policy points.

“I don't see it any differently than Vice President Biden going after 'Medicare for All,'” said Larry Cohen, a longtime Sanders ally and chairman of Our Revolution, the organization that originated from Sanders's 2016 campaign. “It should be about the issues, and when we have differences and believe if something is better, we should say so.”

Biden has made electability a core part of his candidacy, opening the door for Sanders and other candidates to argue that they, and not Biden, are actually the best nominee for the party to draw support in the general election.

The Sanders campaign also takes offense at the notion that Sanders didn't help Clinton in 2016.

In September, October and November of 2016, Sanders held 39 rallies in 13 states on behalf of Clinton's campaign. He did 17 events in 11 states during the last week alone, they point out.

Reines argues that Sanders “needs to either accept the plain fact — supported both quantitatively and anecdotally — that his supporters did not go all-in for Hillary.”

He said Clinton supporters in 2008 offered far more support to then-Sen. Barack Obama Barack Hussein ObamaThe Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon Trump appointees stymie recommendations to boost minority voting: report Obama's first presidential memoir, 'A Promised Land,' set for November release MORE (D-Ill.) after their tough primary than Sanders supporters offered to Clinton in 2016.

“Giving him the benefit of the doubt about his intentions, he still needs to be mindful that at some point he might need to rally millions of people to support someone else. That’s not easy. You can’t just flip a switch. It’s a process. Your supporters have to believe you’re being genuine in your endorsement, and you have to convince them,” Reines said.

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Sanders is rising in polls and increasingly is seen as a real contender in the Iowa caucuses and New Hampshire primary — and for the general election. A new poll released Friday found Sanders with 20 percent support and Warren in second with 17 percent, just ahead of former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq A socially and environmentally just way to fight climate change MORE (D) with 16 percent and Biden with 15 percent.

Some of the Democratic angst about his rhetoric seems linked to the idea that he could actually win.

“Bernie Sanders is the Democratic Party’s version of Donald Trump. Thank god we are smart enough to stop him,” said Democratic strategist Michael Trujillo, who served as an aide to Clinton.

Biden’s support for the Iraq War has come under more scrutiny given the U.S. airstrike that killed Iranian Gen. Qassem Soleimani in Iraq.

And it’s not just Biden who has mentioned it. Warren has also criticized Biden’s vote.

Sanders has generally been a bit more pointed in his criticism, however. In an interview with The Washington Post last week, he said Biden’s support for the Iraq War would make him a weaker general election candidate.

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“It’s just a lot of baggage that Joe takes into a campaign, which isn't going to create energy and excitement,” Sanders said. “He brings into this campaign a record which is so weak that it just cannot create the kind of excitement and energy that is going to be needed to defeat Donald Trump.”

Democratic strategist Eddie Vale said he’s “not as worried as everyone else” about Sanders's rise and rhetoric. But Vale does worry that the primary may become so contentious that unity will be hard to come by on all sides.

“If someone more on the left wins the nomination, it will be equally damaging if all the more moderate folks who complained about Bernie supporters last time in the general [election] do the same thing,” he said.

Democratic strategist Adam Parkhomenko, who served as an aide to Clinton during the 2016 election and an adviser following the election, said he worries that Sanders’s rhetoric will only cause his supporters to stay home again.

“We’d all be better served if he focused his disgruntled energy on Donald Trump,” he said.