In a traditional campaign, said Howard Dean, the former Vermont governor who ran unsuccessfully for president in 2004, “You get to be the frontrunner, then everybody else makes sure you get taken down.”

The difference this year, he said, is that “everybody’s the frontrunner.”

The rancor has resulted in a new round of hand-wringing by Democrats fearful of weakening the party ahead of the general election — and by progressives fearful of weakening their standing against more moderate Democrats in the primary.

Citing their alarm over the recent sniping between the Sanders and Warren presidential campaigns, the liberal grassroots group Democracy for America implored the two to work together and stop attacking each other — unlikely, as a new rift opened over a CNN report that Sanders had told Warren privately in 2018 that he did not believe a woman could win the presidency in 2020.

After Sanders’ campaign disputed the account, Warren appeared to confirm it, saying in a prepared statement that Sanders “disagreed” with her belief that a woman could win.

“Progressives will win in 2020, but only if we don't let the corporate wing or Trump divide us,” Democracy for America said Monday in a tweet.

On the eve of the last debate before the Iowa caucuses, similar concerns were evident on more centrist ground as well. William Owen, a Democratic National Committee member from Tennessee who has endorsed Biden, said Monday that he is “greatly worried” about party unity after the primary campaign.

“I think the candidates should concentrate and focus their attention on the real problem, and that’s Donald Trump,” he said, “not criticize each other.”

Yet Democrats are notoriously worrisome — about the quality of their candidates, about fundraising, about the electoral strength of Trump. And bellicosity is not unusual in the final stretch of a campaign.

In fact, it was not the skirmishing itself, but the scattershot nature of the hostilities, that marked a new turn in the race. Defined until recently by an infrequent series of one-on-one confrontations, the contest is now becoming a melee.

Three weeks before the state’s caucuses, four Democrats are running within 5 percentage points of one another in Iowa, according to the latest Des Moines Register/Mediacom/CNN survey. Behind them, a whole swath of candidates is polling below the state’s viability threshold — each with supporters to offer the frontrunners on caucus day.

Sen. Cory Booker’s departure from the race on Monday opened another small well of support to remaining contenders. In addition, ideological lanes that once seemed likely to define much of the primary’s conflict remain blurred, expanding every candidates’ pool of potential targets.

“We’re in a stage where people are in a civilized world say that they’re highlighting contrast, which is a polite way of saying attacking the stuffing out of each other,” said Philippe Reines, a longtime Hillary Clinton confidant.

The new, multi-directional quality of the campaign came into focus in recent days. First, Sanders intensified his criticisms of Biden, chastising him for his 2002 vote for the war in Iraq and on issues of Social Security and bankruptcy protections. The Sanders campaign believes it can draw some older, working-class voters from the former vice president’s ranks of supporters.

Then, over the weekend, POLITICO reported on a script that Sanders’ campaign produced for volunteers confronting supporters not only of Biden, but also Warren and Buttigieg. Warren supporters, according to the script, could be told she is the candidate of “highly-educated, more affluent people who are going to show up and vote Democratic no matter what” and that “she's bringing no new bases into the Democratic Party.” Sanders’ campaign went after Buttigieg for his lack of support among black voters and young people.

On Sunday, Warren shot back, saying she was “disappointed” that Sanders was “sending his volunteers out to trash me.” But in a jab at Sanders’ last presidential campaign, she warned the party could not abide a repeat of “the impact of the factionalism in 2016."

Left unsaid was Warren’s own willingness to criticize her competitors. Just last month, she lit into Buttigieg for “a fundraiser that was held in a wine cave full of crystals and served $900 a bottle wine,” touching off the “wine cave” cycle of the campaign.

The back-and-forth-and-back again delighted Republicans, including inside the White House, where Trump suggested in a tweet on Monday that Sanders was criticizing Warren because “everybody knows her campaign is dead and want her potential voters.”

With Warren “very angry at Bernie,” he teased, “Do I see a feud brewing?”

The aggressions and mini-aggressions are poised to gain new air on Tuesday night, when the candidates debate in Des Moines. But there is risk in attacking too sharply. Kamala Harris and Julián Castro, who have both dropped out of the race, did not enjoy sustained bumps after issuing withering critiques of their rivals in past debates.

Intra-party attacks, said Gary Hart, the former Colorado senator and two-time presidential candidate, “are never productive and only provide fodder to the other side.”

“People are tired of this,” Hart said in an email, adding that he regretted criticisms of former Vice President Walter Mondale in the Democratic primary in 1984, when Mondale went on to lose in a 49-state landslide. “But it seems to be what the new generation of ‘strategists’ promote by way of justifying their high retainers. Tearing the opponent down rarely if ever adds followers to oneself.”

In the 2004 campaign, a feud between Dean and Dick Gephardt was credited in part with John Kerry’s surge to a surprise victory in Iowa. Dean said Monday that “when you have a multi-candidate race and people are ripping each other apart, then it hurts the ripper and the rip-ee.”

Still, he said, this year, “The honest truth is, I think the candidates have been pretty gentle with each other.”

For a late-stage campaign, Dean said, “This is standard.”