The Democratic establishment, already on edge after Bernie Sanders’ success in Iowa and New Hampshire, is freaking out in the wake of his resolute victory in Nevada’s primary, which showcased not only his snowballing momentum but also an increasingly diverse coalition of supporters. “Today is the most depressed I’ve ever been in politics,” Matt Bennett, of the center-left group Third Way, told Politico following Sanders double-digit win.

With a surging Sanders and a bottleneck of a center lane, the party establishment is facing the very real possibility that it could be running a democratic socialist at the top of its ticket in November—something it fears would lead to Donald Trump's reelection and ruin them in down-ballot races. “In my 30-plus years of politics, I’ve never seen this level of doom,” Bennett added. “It’s this incredible sense that we’re hurtling to the abyss. I also think we could lose the House. And if we do, there would be absolutely no way to stop [Trump].”

Sanders has expanded the movement he began in 2016, generating more enthusiasm than any of his rivals and faring better with minority voters than he did the last time around. But a primary race is different than a general election, and the prevailing wisdom among the establishment is that a more moderate candidate would stand a better chance at beating Trump. Sanders, the thinking goes, is too radical to win over voters in the middle, and his name at the top of the ticket would allow Republicans to brand every candidate with a D next to their name in November as a socialist, just like him. “Before we rush to nominate Senator Sanders in our one shot to take on this president,” said Pete Buttigieg, one of Sanders’ moderate rivals, “let us take a sober look at what is at stake for our party, for our values and for those with the most to lose.”

With Sanders rising, Buttigieg, Joe Biden, Amy Klobuchar, and Michael Bloomberg have all been sharpening their attacks, hoping to slow his momentum and to position themselves as the more realistic, pragmatic alternative to this pie-in-the-sky progressive. Bloomberg launched an ad Monday critical of Sanders‘ record on guns and reportedly plans to aggressively target him at Tuesday night’s debate.

“I ain’t a socialist, I ain’t a plutocrat,” said Biden, who finished second, albeit by a distance, in Nevada. “I’m a Democrat.” Biden’s 2020 hopes may depend on his success in South Carolina, a diverse state that has been seen as his firewall. But Sanders has appeared to cut into the former vice president’s lead there in polling averages, and divisions among moderates—none of whom seem like they’ll be dropping out before Super Tuesday—could allow Sanders, without a clear counterweight, to continue his surge largely unchallenged.

That’s a gloomy prospect for the establishment, whose concerns about Sanders go beyond his perceived electability. Many on the center-left view parts of his agenda, like Medicare for All, as not just politically divisive, but ill-advised. Elizabeth Warren, the race’s other unabashed liberal, has at least made a point to couch her progressive proposals in detailed plans. For all Sanders’ grand promises, he can come off as unconsidered or even evasive when it comes to the specifics of how he’d deliver and what the broader impact would be. Speaking on 60 Minutes Sunday evening, the Vermont Senator offered vague descriptions of how much his health care plan would ultimately cost and how it would be paid for. “It is possible to estimate the costs of your policies if you want to be honest with the American people,” Stef Feldman, Biden’s policy director, tweeted. There are also concerns among Democrats about where his loyalties lay. An Independent senator, Sanders—and his most ardent supporters—has sent strong signals that his main allegiance lies with his own populist movement rather than the Democratic party. “I’ve got news for the Republican establishment. I’ve got news for the Democratic establishment,” he tweeted Friday, on the eve of the Nevada primary. “They can’t stop us.”