With Bernie Sanders largely sequestered in Washington, D.C., at the end of last month ahead of the critical Iowa caucuses, the Vermont senator’s campaign deployed what is arguably its greatest weapon: Alexandria Ocasio-Cortez. And traversing the state, Ocasio-Cortez delivered, drawing massive crowds and pushing the message of progressive populism and younger voters that underlies Sanders' campaign. Ocasio-Cortez’s celebrity is bigger than politics, delivering a wattage that few campaign surrogates can match. She’s the moment’s definitive leader of youth and the left, often mentioned as a possible candidate for the Senate, or even the presidency in 2024. Ocasio-Cortez has undeniably been a boon to the Sanders campaign—she endorsed him in October, as he recovered from his heart attack, which was crucial to him regaining his momentum. But Ocasio-Cortez’s star power and independence make her a wild card. In Iowa, while AOC didn’t exactly go rogue, her performance stoked some tensions between herself and the Sanders campaign.

Following Ocasio-Cortez’s three-day stint, Sanders’s campaign manager, Faiz Shakir, texted AOC’s campaign manager to express his dissatisfaction with aspects of her performance, according to a source familiar with the exchange. Specifically, the Sanders campaign was miffed that Ocasio-Cortez didn’t mention Sanders by name when she closed out a campaign event at the University of Iowa on a Friday night at the end of last month—a fact that Fox News picked up on. (The Sanders campaign noted that this was one of the first campaign events Ocasio-Cortez did without Sanders, who she typically would introduce at the end of her remarks but he was in Washington, D.C. for the impeachment trial. Any reminder to mention the Senator was “good natured ribbing,” according to the campaign.)

Ocasio-Cortez’s comments about the abolition of U.S. Immigration and Customs Enforcement and Customs and Border Patrol were also seen by some within the Sanders campaign as going too far, straying from Sanders’s stance on the issues in encouraging people not to cooperate with law enforcement agencies, according to this source. During an event in Ames, Iowa, Ocasio-Cortez told the crowd, “Organizing is about tipping people off if you start to see that ICE and CBP are in communities to try and keep people safe.” She continued, “I’m not here to reform some of these systems when we talk about immigration. I’m here because Senator Sanders has actually committed to breaking up ICE and CBP. That’s why I’m here.” (Ocasio-Cortez’s campaign declined to comment.)

Abolishing ICE is a major platform plank for AOC and a number of other progressive figures—including Congresswomen Ilhan Omar and Rashida Tlaib, who have also endorsed Sanders. In the past, Sanders has been for stricter immigration laws than those supported by some of his progressive colleagues, though he’s tacked left more recently. He has previously drawn criticism for comments about “open borders.” His ambitious immigration plan now calls for the breakup of CBP and ICE and for the agencies’ current functions to be delegated to other federal agencies, including the State and Treasury departments, as well as the full demilitarization of the southern border. But immigration can be a hot button in states like Wisconsin, Pennsylvania, and Michigan where Democrats have to win, and some felt that AOC had gone off script. (The Sanders campaign said Ocasio-Cortez was not off message and how she talks about these issues is how Senator Sanders talks about these issues.)

Ocasio-Cortez and her team, after all they’d done for Sanders, were said to be annoyed at being called on the carpet. And AOC’s team is also said to have had concerns about radio host Joe Rogan’s controversial unofficial endorsement of Sanders. The Thursday before Ocasio-Cortez’s whirlwind tour of Iowa, Sanders tweeted a video clip of Rogan in which the libertarian-leaning host and comedian said he would “probably vote for Bernie.” “Him as a human being, when I was hanging out with him, I believe in him, I like him, I like him a lot,” Rogan said on an episode of The Joe Rogan Experience. “What Bernie stands for is a guy—look, you could dig up dirt on every single human being that’s ever existed if you catch them in their worst moment and you magnify those moments and you cut out everything else and you only display those worst moments. That said, you can’t find very many with Bernie,” Rogan continued. “He’s been insanely consistent his entire life. He’s basically been saying the same thing, been for the same thing his whole life. And that in and of itself is a very powerful structure to operate from.”