Bernie Sanders is on a mission to bring his "Medicare for all" health care plan to the American people, and it looks as though he is going to make support for single-payer a litmus test for Democratic candidates in 2018.

Sanders and his radical-left legions still dominate the party, pulling Democrats farther and farther to the left while the country remains moderately conservative.  Many Democrats are terrified at the prospect of having to support the unpopular idea of a totally government-run health plan just to get past a primary challenge, only to lose in the November election.

Politico:

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."

But some Democrats believe that pressuring incumbents to support single-payer is more important than winning.

"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."

With that kind of attitude among Sanders supporters, Democrats are going to fall far short of winning a majority in 2018 regardless of how much the GOP is in disarray.

Indeed, the Sanders "revolution" has only just begun.  The senator himself may not run in 2020.  But you can bet that a Sanders clone will be on the ballot if he doesn't.  The tilt of the party to the hard left and the infighting that Sanders supporters are willing to engage in could make the GOP civil war almost irrelevant to the outcome of the midterms.

The energy, the passion, and most importantly the money that Sanders supporters can bring to a campaign will be vital to Democratic challengers and incumbents in 2018.  But winning a Democratic primary by supporting single-payer will expose a huge vulnerability that Democrats will find it hard to overcome.