Many Democrats say they respect the support behind Sanders but fear a self-described Democratic socialist would cede must-win battlegrounds to Trump.

“Swing states have a higher concentration of swing voters. We need a nominee who draws them to the Democratic column,” said Rahm Emanuel, the former Chicago mayor and chief of staff to Obama. “Sanders’ theory, like Trump’s for the right, is based on the notion of a higher turnout of infrequent voters.”

Sanders is in a strong position, based on recent polls, to win Iowa and then New Hampshire. A one-two punch in the first two states could make him hard to stop. Joe Biden’s firewall — his popularity among African-Americans in South Carolina and other Southern states — would face a severe test.

But 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.

Their bind unfolded in plain sight last week when Hillary Clinton complained that “nobody likes” Sanders and wouldn’t initially commit to backing him if he were the nominee. (She later clarified that she would do so.) Clinton suffered a backlash, and Sanders’ has only gained strength in recent days.

That’s largely why Bennett’s push for organized opposition to Sanders isn’t gaining traction.

“We’re not wanting to put our finger on the scale in any way,” said a board member of Priorities USA, the Democrats’ largest super PAC.

Sanders struggled to break out in polls for months in 2019 as fellow progressive Elizabeth Warren mounted a strong campaign. But Warren regressed after she was besieged with questions over how she would pay for Medicare for All — ironically Sanders’ signature issue, but over which he had not been pressed for similar specifics, critics said. (Last week, he again gave a vague answer about its costs to CBS’ Norah O’Donnell.)

Bennett blamed the media for not holding Sanders to the same standard as Warren.

“They let him get away with murder,” he said. “They let him bluster past hard questions.”

Sanders campaign officials did not respond to requests for comment.

Sanders’ resurgence began late last year. After suffering a heart attack, and leaving many to wonder if he would be forced from the race, he received a major boost from liberal darling Alexandria Ocasio-Cortez.

A raft of polls out this weekend had Sanders leading or in second place in Iowa, vying with Biden for the top spot. The enthusiasm for Sanders was on display in Iowa on Sunday, when 1,100 people, according to his campaign, turned out to a rally in Sioux City co-hosted by Ocasio-Cortez and Michael Moore.

“He’s in a position to win Iowa and New Hampshire at this point,” said Ben LaBolt, Obama’s former national press secretary. “Now’s the moment. We’re a week out from Iowa. It might be too late for some states already. But it’s not too late for the nomination.”

LaBolt said Sanders’ competitors should have hit him harder on the debate stage and on the campaign trail. He argued there’s less risk of going negative now that the field has narrowed to a four or five viable candidates in Iowa and New Hampshire.

He added, “There should be a paid media strategy that would challenge him.”

Who should do that? “I don’t have the answer to that.”

Democrats face the same trick box that Republicans faced in 2016. Attacking Sanders only gins up his already enthusiastic base.

“We’re not changing our game plan,” said a Biden associate familiar with the campaign’s strategy. “We don’t feel a need to attack Bernie or anybody else.”

The Biden super PAC has also refrained from attacking the former vice president’s rivals and is not expected to level attacks against Sanders in the days before Iowa.

LaBolt argued that Trump is pushing for Sanders to win, pointing to a recent tweet from the president alleging that the Democratic Party was “rigging the election against Bernie Sanders” to try to help Biden. On Facebook, the Committee to Defend the President, a pro-Trump dark money group is currently running ads against Biden.

Iowa Attorney General Tom Miller, who recently shifted his support from Steve Bullock to Biden, argued that Sanders would harm down-ballot Democrats in battleground districts. Miller pointed to Sanders’ support for Medicare for All and universal free college as issues that could damage other Democrats on the ballot.

“I think there's a concern among some, and I think it's fairly widespread,” he said, “that if Bernie is the nominee he may well lose and take other Democrats down with him.”

Maya King contributed to this report.