“I do believe he can beat President Trump,” said Sen. Kirsten Gillibrand (D-N.Y.), who sought the presidential nomination earlier this cycle. “What Bernie has shown us until now is that he has a very broad base of very, very passionate followers. That is the first thing you need for a campaign on any level. Especially in a red or purple district.”

“He's running even with [Trump] in the national polls and… his win in Nevada shows that he won over all the demographics. So I think he's looking really strong,” agreed Democratic Sen. Tom Udall of New Mexico, a state that Trump wants to flip in November. “It looks like he’s the leader right now and he’s doing very well.”

Should he win the presidency, Sanders’ agenda is sure to run into major roadblocks in the Senate, and the Vermont independent could pose a problem for the party’s efforts to win Senate races in Sun Belt states like Arizona, North Carolina and Georgia. Most Senate Democrats have not endorsed a 2020 candidate, and there’s no apparent move by senators to get behind Sanders while the primary still plays out.

But few in the caucus are predicting the electoral doom that you might hear from their colleagues in the House if Sanders ends up coming out on top in the primary. Summing up the mood of House Democrats running in Trump-won districts with Sanders atop the ticket, Rep. Juan Vargas (D-Calif.) put it this way earlier this month: “We’re going to get absolutely wiped out.”

"I hope I'm wrong, but Bernie seems to have declared war on the Democratic Party,” said Rep. Josh Gottheimer (D-N.J.), a centrist Democrat who has endorsed former New York Mayor Mike Bloomberg. “To win in November, we need someone who will bring us together under a big tent and protect the Democratic majority in the House — not burn the house down.”

That handwringing is enough for Sanders to send out a text message on Monday claiming that “the establishment is in full panic mode.” But such sentiment is not felt in the halls of the Senate, which effectively remains the power center of the establishment of both parties.

“I’m not part of the collective freakout, if that’s what you’re asking,” said Sen. Chris Van Hollen (D-Md.), who chaired the campaign committees for Democrats in both the House and Senate.

In interviews with more than a dozen Democratic senators on Monday, the majority said Sanders was not the electoral anchor that critics are making him out to be. Sen. Maria Cantwell (D-Wash.) said were he to emerge from the long primary as the nominee, Sanders “steals” some voters from Trump’s base: “He is getting a lot of people riled up and he does take some of [Trump’s] votes.

“I absolutely think Bernie can beat Trump,” said Sen. Chris Murphy (D-Conn.), who said several of the other Democrats in the race could also win a general election. “He’s done very well, I think his coalition is real and it’s impressive. But it’s got to stand up to more than just three elections.”

Both Murphy and Cantwell, like many of their colleagues, said with 47 states, the District of Columbia and several territories to go, it was too early to say that Sanders is now the presumptive nominee. But some Democrats are already mobilizing against Sanders amid worries that the anti-Sanders vote is currently fractured among Sen. Amy Klobuchar of Minnesota, former Vice President Joe Biden, former South Bend, Ind., Mayor Pete Buttigieg and Bloomberg.

On Monday, Sanders faced a torrent of attacks from Florida Democrats for praising Cuba's literacy rates under former communist leader Fidel Castro. Sen. Robert Menendez (D-N.J.), a Cuban American, said of Sanders: “If that’s going to be his foreign policy … then we’re doomed at the end of the day.”

“Those who live in my state suffered enormously under the regime, the more than a million people who fled. I’m sure they all think that the literacy program was worth all of that,” Menendez said.

Menendez has previously criticized Sanders for not denouncing Venezuelan strongman Nicolás Maduro. And many of Sanders’ positions have been well-litigated since his run against Clinton in 2016, on issues from guns to immigration to his support for President Barack Obama. None of it has slowed him down in the Democratic primaries, and he remains among the most competitive challengers to Trump in public polling.

Sanders isn’t even technically a Democrat, preferring to keep his “independent” label as a senator even as he pursues the Democratic nomination and serves on Senate Minority Leader Chuck Schumer’s leadership team.

Sanders’ embrace of socialism delights Republicans and concerns moderate Democrats like Sen. Joe Manchin (D-W.Va.), who calls himself a “compassionate capitalist.”

But Sanders’ colleagues said GOP attacks on progressives are nothing new and won’t do much if Sanders is the nominee.

“Republicans have been using labels like that ... for the last 50 years. That’s what they said Medicare was,” said Sen. Martin Heinrich (D-N.M.). “I don’t think it’s substantively any different from the way they’ve campaigned forever.”

The angst among Democrats carries some echoes of the 2016 GOP primary, when Republican insiders thought Trump was a disastrous candidate only to win the White House. Appearing on CBS, Sen. Tim Scott (R-S.C.) said on Monday that Sanders has a similar “outsider game” to Trump’s and said he’s the toughest potential opponent for the president.

Now, the conventional wisdom is that Sanders is going to run away with the Democratic nomination, then lose to Trump while putting the House Democratic majority at risk and blowing a shot at winning the Senate. But it’s not a popular view among the lawmakers that know Sanders best.

“He’s doing well. His following is very strong in terms of turnout, energy and you have to be impressed about how many young people are attracted to our 78-year old colleague,” said Senate Minority Whip Dick Durbin (D-Ill.). “I think he could [beat Trump]. I don’t know if that will be the outcome. Too early to predict.”

Sarah Ferris contributed to this report.