Bernie Sanders was off the campaign trail over the weekend, stuck in the Senate for impeachment hearings, but if polling is any indication, that didn’t stop his campaign’s momentum. A New York Times/Siena College poll of Iowa Democratic voters shows 25% of respondents would vote for Sanders in the Iowa caucus on Feb. 3. A poll from Boston’s Emerson College of Iowa Democrats and independents found 30% of respondents planning to vote for him.

Along with Sanders’ growth in the polls, however, comes a growth in backlash, primarily from centrist Democrats. As Politico reported Monday, they “fear a repeat of 2016 is in the making — when mainstream Republicans scoffed at the idea that Donald Trump could ever win the nomination, until he became unstoppable — only this time from the left.”

In interviews with multiple outlets, Democratic establishment figures are attacking Sanders on multiple levels. Last week, Hillary Clinton homed in on his relationship with fellow party members, telling The Hollywood Reporter, “Nobody likes [Sanders], nobody wants to work with him, he got nothing done.”

Rahm Emanuel, former mayor of Chicago and chief of staff to President Obama, expressed concern about Sanders’ ability to attract swing voters, telling Politico, “Swing states have a higher concentration of swing voters. We need a nominee who draws them to the Democratic column.”

Iowa Attorney General Tom Miller believes Sanders will be harmful to down-ballot Democratic candidates in swing districts. He told Politico, “I think there’s a concern among some, and I think it’s fairly widespread … that if Bernie is the nominee he may well lose and take other Democrats down with him.”

The New York Times on Monday focused on concerns about Sanders’ supporters, who have been accused of misogyny, racism and extreme online bullying. Bakari Sellers, a South Carolina state legislator, told the Times, “You have to be very cognizant when you say anything critical of Bernie online. You might have to put your phone down. There’s going to be a blowback, and it could be sexist, racist and vile.”

Matt Bennett, co-founder of the centrist group Third Way, believes Sanders has gotten off easy in the media. “They let him get away with murder,” he tells Politico. “They let him bluster past hard questions.”

When it comes to pushing back against Sanders’ campaign, many centrist Democrats say they are in a bind. According to Politico, “The Democratic establishment is caught in a catch-22: Attack Sanders and risk galvanizing his supporters and turning him into a martyr of the far-left. Or leave him alone and watch him continue to gather momentum.”