INDIANOLA, Iowa — Hillary Clinton has been leveling attacks on Sen. Bernie Sanders as he gains steam heading into the Iowa caucuses, prompting him to try to quell the conflict and manage his more combative supporters who are inclined to hit back.

The strains were on vivid display Friday and Saturday as Rep. Rashida Tlaib (D-Mich.), a top Sanders surrogate, booed a mention of Clinton’s name onstage at a campaign event. Hours later, she expressed regret for her behavior — leading Sanders’s campaign manager to send her a supportive message on social media: “Rashida, you’re all good.”

This raises the prospect that the primary could plunge into darker territory in coming weeks, as Clinton appears determined to warn her party that Sanders is not the unifier they need and his backers respond with fury. And that is alarming Democrats who say the party must set aside its divisions to defeat President Trump in the fall.

“I’m not really on board with the in-house Democratic bickering,” said Joshua Fox, 44, a Sanders supporter who attended a campaign event here for the senator on Saturday. “I think we need to get together.”

That sentiment was echoed on Twitter by Sen. Brian Schatz (D-Hawaii), who wrote, “It is 2020 not 2016 so please everyone act accordingly.” He added in a second tweet, “Also, I mean EVERYONE.”

Other Democratic candidates, sensing voters’ angst over a Clinton-Sanders feud that will not die, even now that the party seeks to focus on moving beyond the last election, have been trying to take advantage by playing themselves up as unifiers.

“I didn’t much enjoy, as a Democrat, living through the experience of 2016. And I want to make sure 2020 resembles 2016 as little as possible,” said Pete Buttigieg, former mayor of South Bend, Ind.

Today’s Clinton-Sanders dynamic is in some ways the reverse of 2016. Four years ago, Sanders was a rebel with little to lose from taking shots at the party establishment; this time, he’s a leading contender with more at stake and an interest in keeping the peace, and it’s Clinton who does not risk much by settling old scores.

Sanders on Saturday sought to dispel the notion that he is not a team player.

“Certainly, I hope that we’re going to win. But if we do not win, we will support the winner,” Sanders said, returning to a line he has often used. “And I know that every other candidate will do the same. We are united in understanding that we must defeat Donald Trump.”

Such comments have not convinced Clinton, who has suggested that she thinks Sanders’s behavior contributed to her defeat. In recent weeks, she has bluntly criticized her former rival, accusing him of not doing enough to help her after she won the primary.

In an interview for the podcast “Your Primary Playlist” published Friday, she was asked what Sanders could do to unify Democrats against Trump. “Well he can do it, for one,” Clinton said with a laugh, emphasizing “do it.” “That’s not our experience from 2016.”

That followed the release of a new documentary about Clinton in which she says of Sanders, “Nobody likes him, nobody wants to work with him, he got nothing done.” Asked later by the Hollywood Reporter if that assessment still stands, Clinton nodded and said, “Yes, it does.”

Sanders has shown little appetite to fan the flames. “On a good day, my wife likes me, so let’s clear the air on that one,” Sanders joked, before redirecting the conversation to Trump’s impeachment.

Humor aside, Sanders has a lot riding on his ability to quash any notion that he’s ill-equipped to unify the party, and his campaign sought to lower the temperature on Saturday.

“As Democrats, we have a responsibility to engage in debate with respect, civility, and empathy, modeling the best of what our nation can be,” said Rep. Ro Khanna (D-Calif.), a national co-chair of the Sanders campaign. “Let that be common ground for both Clinton and Sanders supporters.”

But Sanders’s supporters can be an independent lot, and many say that they will not take their cues from him and prefer to hit back.

The kerfuffle over Tlaib’s comments began when she appeared in a panel discussion in Clive, Iowa, on Friday evening, where a moderator raised the topic of Clinton. When some in the audience groaned, the moderator said, “No, we’re not going to boo.”

Tlaib interjected, “No, no, I’ll boo,” and proceeded to boo emphatically. Some in the crowd cheered, and Reps. Pramila Jayapal (D-Wash.) and Ilhan Omar (D-Minn.), seated near her onstage, smiled.

“That’s all right — the haters will shut up on Monday,” Tlaib added.

Sanders was not present at the event, though it was on behalf of his campaign. Taking to Twitter on Saturday, Tlaib backpedaled.

“In this instance, I allowed my disappointment with Secretary Clinton’s latest comments about Senator Sanders and his supporters get the best of me. You all, my sisters-in-service onstage, and our movement deserve better,” Tlaib wrote on Twitter.

She added: “I will continue to strive to come from a place of love and not react in the same way of those who are against what we are building in this country.”

Sanders campaign manager Faiz Shakir then sent his “Rashida, you’re all good” tweet, adding, “We love your passion and conviction. Don’t change.”

Clinton spokesman Nick Merrill took issue with what he said was a suggestion that Tlaib had no reason to regret her action.

“I can’t imagine this kind of behavior is something Iowans want to see from candidates and their surrogates,” Merrill said. “And I don’t imagine the vast majority of voters in Congresswoman Tlaib’s district, which Secretary Clinton won by over 60 points in 2016, want to see this either.”

The icy dynamic between Clinton and Sanders has caught the attention of other campaigns. In different ways, Sanders’s top rivals — Buttigieg, Sen. Elizabeth Warren (D-Mass.) and former vice president Joe Biden — have focused their closing arguments on a promised ability to unite the party and the country.

At an event Saturday, Warren, who has tangled with Sanders herself, spoke in front of signs reading, “Unite the party.” At another campaign stop, she sought to put Tlaib’s comment in perspective. “I understand that during primaries, people can get heated,” she said.

She then pivoted to her new theme as a unity candidate, encouraging supporters to see her as the Democratic hopeful best able to bind a fractured party. “What’s important is, we come together as a party because we have one really important job, and that is to beat Donald Trump,” Warren said. But some of Sanders’s allies are not backing away. At campaign stops in Iowa last weekend, filmmaker Michael Moore, who introduced the candidate, brought along a printed list of campaign events Sanders held for Clinton after losing to her in 2016, a pointed effort to rebut the notion that he did little to help her.

And from Clinton’s side, there were few signs she would ease her criticism anytime soon.

“Her larger point is you’ve got to work toward unity — it’s not handed to you,” said Philippe Reines, a longtime Clinton adviser and confidant. “And he showed in 2016 he is not capable of going all in.”

Jenna Johnson in Des Moines contributed to this report.