RoseAnn DeMoro, a close friend and staunch supporter of Bernie Sanders, has been so stinging in her criticism of Sanders’s rivals and detractors that his campaign has called her and asked her to “tone it down,” she said.

The former head of an influential nurses union isn’t letting up.

“We’re not programmed,” DeMoro said in an interview. “There’s no lockstep here.”

As Sanders (I-Vt.) has risen in the polls and clashed more sharply with his opponents, a dilemma has emerged for his campaign — how to rein in his fervent following and merge it with a Democratic Party he and his supporters have frequently fought.

It’s become a central question for the Sanders candidacy, as many Democrats are desperately seeking a nominee who can unite the party against President Trump — and it’s one his campaign is addressing with new urgency.

Campaign officials have been pressing surrogates and other allies not to escalate combustible disputes that have recently become more personal, according to people with knowledge of the situation, including a tense fight with Joe Biden, a separate altercation with Sen. Elizabeth Warren (D-Mass.) and a renewed schism with Hillary Clinton.

Sanders issued a rare apology this week to Biden after a prominent supporter wrote an op-ed declaring that the former vice president has a “big corruption problem.” And after Clinton slammed Sanders, he sidestepped her comments instead of rebuking her.

But many Sanders supporters, and even some campaign aides, are inclined to take a more aggressive approach that they say will highlight important differences. DeMoro said that Biden has a “character issue,” and she sarcastically thanked Clinton for drawing “a lot of neutral people” to Sanders with her attacks.

Zephyr Teachout, the Sanders supporter who wrote that Biden had corruption troubles, defended her op-ed in an email to The Washington Post, suggesting that the issue of corruption remains crucial in deciding which Democrat is best to take on Trump.

“We can’t let Trump and Trumpism make us stop talking about systemic corruption because he has abused the term, and tried to destroy its meaning, and because his own corruption is so grotesque,” she said.

Some close Sanders allies acknowledged the difficulty of balancing the passions of his loyal backers, inspired by his calls for a political revolution and his staunchly liberal platform, with the need to demonstrate a broader unity. The task is crucial, they said, as Sanders seeks to woo Democrats in Iowa, where voters are known for rejecting negative politics, ahead of the state’s Feb. 3 caucuses.

“This is a moment for our campaign to show we are capable of uniting not just our base, but the broader Democratic Party and country — and healing a nation that is deeply divided,” sad Rep. Ro Khanna (D-Calif.), a national co-chair of the Sanders campaign.

Khanna drew parallels to the test Barack Obama faced in 2008 when revelations about racially charged sermons by his pastor, Jeremiah Wright, threatened to derail his campaign. Obama rose to the occasion with a unifying call and by showing leadership, Khanna said, and Sanders has similarly met the moment “by making it clear that everyone in this movement has to live up to his values and standards.”

Adding to Sanders’s challenge are old wounds from his 2016 battle with Clinton that were reopened this week by her scathing assessment of her former rival. “Nobody likes him,” she said of Sanders in a new documentary. She followed that up with comments published Tuesday in the Hollywood Reporter, in which she accused Sanders of supporting sexist attacks. Clinton later sought to soften her remarks.

Sanders opted not to engage her, instead issuing a brief written statement saying his focus was on serving as a juror in Trump’s Senate impeachment trial. Despite being peppered by questions from reporters at the Capitol, Sanders has studiously declined to criticize his former rival.

Sanders supporters such as DeMoro, however, are not holding back, portraying Clinton as a has-been. “I think she must be unaware of how low her favorables are,” DeMoro said. “It really outraged a lot of people. It activated a lot of neutral people. Thank you, Hillary, thank you.”

DeMoro said that although Sanders is often reluctant to stoke personal fights, his supporters want him to throw more aggressive counterpunches. “Bernie’s thing is he doesn’t like the personalization thing — he’s too good for that,” she said. “In the base, if you will, we often feel like Bernie should hit back harder, because we feel insulted by the fact that they take such swipes at him.”

Sanders has long walked a careful line with the Democratic establishment. He’s refused to join the party, calling himself an independent and a democratic socialist. At the same time, he’s worked closely with Democratic leaders in the Senate and almost always voted with them, and in turn, he has received an influential leadership post.

Sanders’s explosive rise threatens that balance. Many Democrats say he has not moved swiftly enough to defuse his supporters’ attacks, which they worry could hurt the party.

“I don’t think he’s done enough,” said Rufus Gifford, who served as finance director for Obama’s 2012 campaign and has raised money for Biden. “I don’t believe, dating back to 2016, that the campaign apparatus has done enough to tamp the negativity down.”

Champions of Sanders disagree. They note that he has frequently pledged to support whomever emerges as the Democratic nominee. He also sent an email to supporters at the outset of his campaign urging them to be respectful of his opponents. Ben Cohen, a national co-chair of the Sanders campaign, said the campaign recently distributed a note reinforcing that approach.

Sanders has often said that he is not in the business of disparaging his rivals in a personal way, preferring instead to draw contrasts over policy. As he put it on the campaign trail in New Hampshire last weekend, “Should we be engaged in ugly personal attacks against each other? I think not.” He added, “Why do you run for office if you don’t talk about your vision and your record?”

But the line between policy critiques and personal attacks is not always clear — especially to his backers — and even as Sanders has sought to tamp down some tensions, he has been stoking others. A fight has intensified this week over Sanders’s claims that Biden has not been a strong defender of Social Security, which Biden forcefully disputes. Each candidate put out a video critical of the other.

In Teachout’s op-ed, which a Sanders aide said the senator did not know of in advance, she argued that Biden is too cozy with big donors from the credit card, health-care and fossil fuel industries, which Sanders routinely attacks on the campaign trail.

“I know it seems crazy, but a lot of the voters we need — independents and people who might stay home — will look at Biden and Trump and say: ‘They’re all dirty,’ ” Teachout wrote.

Sanders adviser David Sirota, who has aggressively gone after Biden on social media, highlighted Teachout’s op-ed in his Bern Notice newsletter. Sirota declined to comment.

Associating Biden with corruption — an attack also leveled by Trump — did not sit well with Sanders. “It is absolutely not my view that Joe is corrupt in any way, and I’m sorry that that op-ed appeared,” Sanders said in a Monday interview with CBS News, hours after the article was published.

Biden told the Iowa television station WHO that Sanders also apologized in person, and he thanked the senator.

Sanders has also criticized the tactics of other supporters. One Sanders aide publicized a video this month that showed clips of Biden talking about Social Security, which fact-checkers said had been edited in a misleading way. Sanders acknowledged Sunday that the video should have had “the whole context,” though he reiterated his case that Biden has not been a strong supporter of Social Security.

In the CBS interview, Sanders made a broad request to his backers: “I appeal to my supporters: Please, engage in civil discourse.” Still, he said, his partisans were not the only ones crossing the line, and he encouraged everyone to “have a debate on the issues.”

The clash between Sanders and Warren focused on a more personal matter, involving a private conversation in 2018. Warren says that when she asserted that a woman could defeat Trump, Sanders disagreed; he forcefully denies having done so.

Sanders has appeared eager to defuse the matter, but once again, the actions of his followers did not always match his message. Scores of social media users identifying themselves as his supporters used snake icons to attack Warren as duplicitous and touted a #Never­Warren hashtag.

Chelsea Janes contributed to this report.