Top Democrats are increasingly alarmed that Sen. Bernie Sanders could gain unstoppable momentum from the primary voting that starts next week. But they also fear that any anti-Sanders effort would backfire, and that has sidelined any significant stop-Sanders effort for now.

Even the hint of an organized anti-Sanders movement would risk alienating the Vermont independent’s sometimes belligerent supporters and play into claims that the process is “rigged,” many Democrats say privately. Democratic House candidates in swing districts say they are nervous about running on the same ticket as Sanders, but they, too, are reluctant to say so publicly.

That is leading some Democratic centrists to warn that the silence carries a risk of waiting until it’s too late.

To understand how Bernie Sanders became a presidential contender, you have to start in Vermont. (The Washington Post)

“People need to start taking Bernie pretty seriously — there is a really substantial risk of him becoming unstoppable if he wins these early states by large numbers,” said Matt Bennett, executive vice president for public affairs at Third Way, a centrist Democratic group.

But despite “some discussions among people wringing their hands,” Bennett said, “it’s not like our phone is ringing from people saying, ‘Let’s do something.’ ”

Third Way blasted out an email to Iowa Democrats on Tuesday saying that Sanders has a “politically toxic background” and “his far left positions will repel swing voters.” And Democratic Majority for Israel, a group of pro-Israel Democrats, has reserved $700,000 in television ads that will begin airing Wednesday, apparently the most concerted effort on television so far to attack Sanders.

But those moves remain the exception within the party. Some Democrats say they erred in 2016 by antagonizing Sanders and his supporters, and they are loath to risk doing so again. Others warn that treating Sanders too gently, however, could lead to a repeat of Republicans’ experience of 2016, when they underestimated Donald Trump until it was too late to stop him.

Either way, the anxiety is palpable.

“If Bernie is on the ticket as the nominee, I have no chance whatsoever,” said one Democratic House candidate in a swing district, speaking on the condition of anonymity for fear of antagonizing Sanders supporters. “And if you wrote that, it would blow me up in the primary.”

The candidate added: “Bernie has a real following. But it’s a minority, and he turns off a whole lot of people.”

This account of the growing angst inside the Democratic Party over the risks of nominating a socialist to take on President Trump is based on interviews with two dozen officials, strategists and leaders. They say Sanders could get crushed by the Trump machine if he were the nominee; at the same time, they fear taking him on would rile up his fierce and loyal army.

Sanders’s supporters — and there are many, including influential figures such as Rep. Alexandria Ocasio-Cortez (D-N.Y.) — argue that the critics have it exactly backward. The Democrats’ real mistake, they say, would be nominating a candidate who fails to excite the base, while a Sanders candidacy would not only energize Democrats but also draw new voters to the party.

But others say that Sanders, for all his success, has never faced tough scrutiny and that the Trump campaign would eviscerate him if he were the nominee, unearthing all manner of dubious or impolitic statements from the 78-year-old senator’s long, unorthodox political career.

“Sanders would be uniquely problematic as the nominee,” said Ben LaBolt, a former aide to President Barack Obama, because he is a “largely untested candidate.”

That point was made even more bluntly in the message Third Way emailed Tuesday to Democrats in Iowa.

“If Bernie Sanders becomes the nominee, Trump’s odds of winning a second term go up dramatically, which is why Team Trump has labeled him their ‘ideal’ nominee,” reads the two-page document. “Iowa Democrats: Please don’t do what Trump wants you to do.”

The 30-second spots from a super PAC connected to Democratic Majority for Israel, meanwhile, feature Iowans explaining their concerns about whether Sanders can win in November.

“I like Bernie. I think he has great ideas. But Michigan? Pennsylvania? Iowa? They’re just not going to vote for a socialist,” a voter named Michael Kuehner says in the ad. Another Iowan, Darby Holroyd, says, “I do have some concerns about Bernie Sanders’s health, considering the fact that he did have a heart attack.”

Sanders had a heart attack in October that required him to be hospitalized and briefly kept him off the campaign trail.

Mark Mellman, president of Democratic Majority for Israel and the aligned super PAC, said his group began planning the campaign several weeks ago when it became clear that Sanders could be the nominee, adding that he has not worked with any other groups.

Though the ads focus on electability, Sanders, who would be the first Jewish president, has drawn the ire of some pro-Israel groups by calling the country’s prime minister, Benjamin Netanyahu, a “racist” and by campaigning with Reps. Rashida Tlaib (D-Mich.) and Ilhan Omar (D-Minn.), who support a controversial Israel boycott movement.

Sanders tweeted a message Tuesday evening blaming the ads on rich and powerful forces that oppose him.

“It’s no secret that we’re taking on the political establishment and the big-money interests, who are now running attack ads against us in Iowa,” Sanders said in the tweet. “But we have the people, and our grassroots movement will prevail.”

Sanders campaign manager Faiz Shakir sent out a fundraising appeal based on the attacks.

“We have a small lead in Iowa heading into Monday’s caucus,” he wrote. “But outside groups are on the attack and hoping to stop us. Bernie needs us all if we’re going to fight back and win.”

Those worried about Sanders have not coalesced around a clear alternative, leaving the centrist vote splintered among a handful of candidates, each of whom has vulnerabilities.

Some voters are concerned about former vice president Joe Biden’s age and acuity — he is 77 — while others worry that former South Bend, Ind., mayor Pete Buttigieg, 38, is untested. If Sen. Amy Klobuchar (D-Minn.) or Mike Bloomberg, the former New York mayor, gains strength, that could further muddy the waters.

Meanwhile, Sanders has shown signs of consolidating support from the party’s liberal wing at the expense of Sen. Elizabeth Warren (D-Mass.). And even Sanders’s detractors grudgingly acknowledge that he is drawing large, energetic crowds and bringing new faces into the party. Republicans have been watching warily, too, concerned that Sanders is tapping into a strain of the electorate similar to the bloc that bolstered Trump four years ago.

“A lot of the party was naive to Bernie’s strengths and didn’t fully realize how well positioned he was until this last week,” said Dan Pfeiffer, a former adviser to Obama. “This is very well set up for Bernie.”

If Sanders scores a strong win in Iowa on Monday, some Democrats say, the week before the Feb. 11 New Hampshire primary could shape up as a crucial window to stifle him, although Sanders decisively won New Hampshire four years ago.

Democratic operatives say initial efforts to find major donors for an anti-Sanders effort have been unsuccessful. Those who have had such talks say it was easier to scare donors about the prospect of a Warren presidency, because they were more prone to believe she might get elected and implement far-reaching policies.

“I haven’t heard of any group of people huddling to say, ‘How can we stop Bernie?’ ” Pfeiffer said. “But I’m positive it would have the opposite effect as intended.”

Democrats worried about a split this fall between the party’s pro- and anti-Sanders factions, no matter who wins the nomination, cite several recent episodes.

Some top Sanders allies, for example, expressed outrage this week at a list of nominations that Tom Perez, chair of the Democratic National Committee, is considering for committees at the party’s convention this summer.

“The DNC should be ashamed of itself, because it really is a slap in the face to folks who were asking for reform,” Nina Turner, the national co-chair of the Sanders campaign, said Monday in an interview with Status Coup. “And if the DNC believes that it’s going to get away in 2020 with what it did in 2016, it has another thing coming.”

And during a mock caucus conducted Monday night in Des Moines, Sanders supporters walked out when it became clear they would not meet the 15 percent threshold needed to earn delegates, rather than lending their strength to another candidate. To some critics, that suggests Sanders backers might abandon the party if Sanders is not the nominee.

Biden, campaigning Tuesday in Iowa, fretted openly about how Sanders’s supporters behaved and whether such an attitude would affect the party’s fortunes.

“You can’t do that kind of stuff,” Biden said, referring to the walkout. “It’s going to make a difference.”

When asked whether the party could unite behind Sanders if he were the nominee, Biden paused for several seconds.

“We have to. I’m not going to make judgments now,” he said. “But I just think that it depends upon how we treat one another between now and the time we have a nominee.”

Still, he said: “I think yes, I think we can unite. We have to unite.”

But plenty of others are worried about whether the party will be able to come together.

“Of course I’m concerned about that,” former senator Barbara Boxer (D-Calif.) said in an interview. “That is a question. If we want to win, we are going to have to unite.”

Such concerns help explain why Sanders’s rivals have largely avoided attacking him — or one another. As he gained momentum, several of his fellow candidates began criticizing him more directly over the weekend, but most of the attacks remained indirect.

One potential vulnerability for Sanders in the Democratic primary could be gun issues, because the party is increasingly coalescing around gun control and Sanders has deviated from that position in the past.

Sanders’s record of supporting some pro-gun measures, including shielding manufacturers from liability, came up repeatedly in the 2016 primary. But even Biden — who makes the issue a standard part of his stump speech — has rarely brought it up this time around.

Similarly, a super PAC supporting Biden — the sort of nominally independent group that is often used to launch attacks — has no plans to criticize Sanders, instead running ads praising Biden’s biography and electability.

A coordinated effort to diminish Sanders could backfire spectacularly, warned Steve Grossman, former DNC chair. It would cause major damage “if it appears in some way, shape or form that there is an attempt to meddle with the process,” said Grossman, who has endorsed Buttigieg.

“Democrats have to attract new voters,” he added. “Those voters — first-time voters, activist voters — would react extraordinarily negatively and might potentially walk away from the party if they felt people were inappropriately interfering.”

Beyond that, some party elders gently ridiculed the idea that the “establishment” could mount a coordinated effort to take down Sanders, even if it wanted to.

“Anybody who thinks the Democratic Party can rig anything — it’s like, okay, whatever,” said Joe Trippi, a Democratic consultant. “The two parties have none of that power anymore. If the Republican Party could have stopped Trump, they would have.”

Still, he added: “It’s good rhetoric. Is the party a good foil? For the Sanders campaign, of course it is.”