House and Senate Democrats have wondered for months whether Bernie Sanders’ supporters might choose to focus their energy on launching primary challenges to party moderates in 2018. They’re about to get an answer.

Sanders has decided the moment is right to launch his proposal for the single-payer health insurance system that helped form the backbone of his presidential message. And Democrats who don’t get behind it could find themselves on the wrong side of the most energetic wing of the party — as well as the once and possibly future presidential candidate who serves as its figurehead.

The Vermont senator himself has not explicitly said he’ll support primary challenges to those who won’t support his push for a so-called Medicare-for-all health care plan. But there are plenty of signs that Sanders and his allies view the issue as a defining moment for Democratic lawmakers.

“Our view is that within the Democratic Party, this is fast-emerging as a litmus test,” said Ben Tulchin, the pollster for Sanders’ White House run.

The single-payer concept is increasingly popular in the party — high-profile senators like Cory Booker, Kirsten Gillibrand and Kamala Harris have expressed some support, and, for the first time, a majority of House Democrats have now signed on to the single-payer bill that Rep. John Conyers has been introducing regularly for more than a decade.

But even as leading party figures have drifted toward supporting a single-payer system similar to the one proposed by Sanders, almost none of them expect anything like it to become law while Republicans control Washington.

With Sanders promising to play a major role in 2018 races, that’s led many party officials to worry about the prospect of his involvement in primaries that could upend the Democratic establishment’s plans to win crucial House, Senate and gubernatorial seats.

The fears are acute enough that when the Nevada chapter of Our Revolution — the political group spawned from the Sanders presidential campaign — endorsed long-shot candidate Jesse Sbaih in the state’s Democratic Senate primary over party favorite Rep. Jacky Rosen, retired former Sen. Harry Reid felt the need to call Sanders directly.

Don’t endorse Sbaih, and don’t let the national Our Revolution group accept its Nevada chapter’s recommendation to back him either, the former minority leader implored his friend. Sanders agreed, said a Democrat familiar with the interaction.

“There’s a concern that [Sanders allied] people will try to make a stir,” said a senior Democratic aide working on a 2018 campaign. “You can’t just be a liberal Democrat in a lot of these states and be elected. [So] the question is how we improve the lives of people instead of playing these political games.”

Sanders allies don’t find that argument convincing.

“Any Democrat worth their salt that doesn’t unequivocally say Medicare-for-all is the way to go? To me, there’s something wrong with them,” said former Ohio state Sen. Nina Turner, president of Our Revolution. “We’re not going to accept no more hemming and hawing. No more game playing. Make your stand.”

Sanders himself has stood alongside Democrats in fights like the recent one against the GOP’s health care plans. He’s toured states with wavering Republican senators to pressure them on the issue and quickly condemned a recent single-payer measure pushed by Republican Sen. Steve Daines as a ploy designed to trick Democrats.

His team has been working with fellow progressive senators to enlist co-sponsors for his measure, said Democrats across Capitol Hill. Within Sanders circles, the increased popularity of single-payer arrangements is seen as a sign that his long-promised “political revolution” is underway.

“He’s been vindicated by the presidential campaign,” said Mark Longabaugh, a senior Sanders 2016 campaign adviser.

The Vermont independent has signaled that he expects serious resistance even from Democrats, but he has yet to spell out how he’ll fight back.

“We will be taking on the most powerful special interests in the country: Wall Street, the insurance companies, the drug companies, the corporate media, the Republican Party and the establishment wing of the Democratic Party,” he emailed supporters last Tuesday.

What’s clear is that Sanders’ large and politically active following has stopped Democrats from confronting him directly — including when it comes to offering alternatives to his Medicare-for-all measure. Many still remember the swift and angry January response from grass-roots progressives including Sanders supporters toward Booker for a symbolic drug importation vote, and toward Sen. Elizabeth Warren for her procedural vote in favor of Ben Carson’s nomination as Housing secretary.

“It represents the broader question of what the Democratic Party stands for, [so] this is a fundamental moment for Democratic senators. It’s an issue that everyone is going to be watching to see how they respond,” said Chuck Idelson, a senior operative for the National Nurses United union, which served as one of the most prominent backers of Sanders’ campaign and has long been a needle in the side of establishment Democrats.

Like many Democrats who are closely aligned with Sanders’ political operation, Idelson stopped short of primary threats. But he refused to rule out the possibility that his group might consider backing challenges of sitting Democratic lawmakers who don’t back the plan.

“Our organization, and plenty of other people out there, are going to be holding the Democrats accountable,” Idelson said. “What are we electing people for if they’re not going to be fighting for getting people health care when they need it?”

Other Sanders-allied progressives have been equally adamant on the need to give his Medicare-for-all push a starring role in forthcoming primaries after the recent Capitol Hill health care fights and the stalling of a much-publicized California state legislative proposal.

“We should run on Medicare-for-all in the 2018 and 2020 elections,” said Bay Area Congressman Ro Khanna, a Sanders backer who has encouraged primary challenges. “The Democrats that are activists are there, the Democratic voters are there, but now we just need enough of the elected officials to listen to where their constituents are.”

The distrust between Sanders forces and the establishment is increasing the tension. Some Democratic senators privately bristled at the health care rallies that Sanders and others organized across the country in January: They were shocked to be greeted by angry Sanders backers in the crowds who loudly urged them to back a single-payer plan, according to several Democratic senators and aides. There is also longstanding grumbling over his refusal to share his campaign email list with other Democrats and, more recently, over his vote against a new round of sanctions against Russia and Iran.

On the other side of the divide, Sanders allies insist the party seldom acknowledges the role of the senator’s 2016 presidential bid in shaping the party’s new agenda, whether on health care, a $15 minimum wage, or free college. And they express frustration that Democratic gatekeepers are still slow to accept Sanders’ likely front-runner role if he chooses to run for president in 2020.

In the words of one senior aide to Sanders’ campaign, “A special cloud of denial formed over the swamp when polls started coming out showing Bernie was the most popular politician in the country.”

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