A surge of Democratic support for socialized healthcare is sparking new questions over whether the issue is becoming a litmus test for Democrats ahead of the 2018 midterm elections.

Single-payer healthcare, or a healthcare system run by the federal government, has largely been on the sidelines of the Democratic Party's policy platform, but that is quickly changing as Sen. Bernie Sanders, I-Vt., gains clout in the party and Republicans struggle to repeal Obamacare.

Sanders is expected to release a new version of his Medicare-for-all single-payer healthcare plan soon. He has pushed for the system for years while in both the House and Senate and touted a similar plan when he challenged Hillary Clinton last year for the Democratic presidential nomination.

While universal healthcare has been a plank in the Democratic Party's platform, party leadership has long shied away from embracing the idea for fear it could cost them support from centrist or independent voters.

But support is growing among the public for the idea. A June poll from Pew Charitable Trusts found that 60 percent of those surveyed believe the federal government is responsible for ensuring healthcare coverage for all Americans, compared with 39 percent who do not.

And signs show that more of the party is embracing the expansion of government-run healthcare.

For the first time, a majority of House Democrats signed on to a Medicare-for-all bill and mainstream lawmakers such as Sens. Cory Booker, D-N.J., and Kirsten Gillibrand, D-N.Y., have showed some support.

But Sanders is keen to tamp down expectations among the liberal base that he energized last year.

He told the Washington Post last week that he does not believe that single-payer is a "litmus test" for potential 2018 candidates.

A Democratic Party official told the Washington Examiner that it doesn't believe in a litmus test for any issue. Party leaders already had to fight back suggestions that they have a litmus test on abortion rights.

While Sanders and the party fight the litmus test label, some liberal groups allied with him haven't gotten the message.

The liberal group Our Revolution, founded to continue Sanders' policy goals, didn't mince words when detailing what role single-payer will play in their selection of candidates to endorse and support next year.

"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," group President Nina Turner told Politico recently. "We're not going to accept no more hemming and hawing."

The new vigor for single-payer comes as Republicans are trying to tie Democrats in tough reelection races to Sanders.

The Republican National Committee several weeks ago sent out a press release detailing that several red-state Democratic senators stuck close to Sanders. For instance, it said that Sen. Debbie Stabenow, D-Mich., voted with Sanders 91 percent of the time and co-sponsored a bill with him to raise the minimum wage to $15 an hour.

Other targeted lawmakers include Sens. Joe Donnelly of Indiana and Sherrod Brown of Ohio.

"Although the red-state Democrats will likely keep their distance from Senator Bernie Sanders this week, each has voted with him an overwhelming amount," the RNC said.

Meanwhile, Republicans have been unable to make good on their seven-year pledge to repeal Obamacare. An effort to pass a "skinny" repeal bill to gut major parts of the law failed 49-51 in late July with all Democrats and three Republicans opposing it.

Numerous Republicans have vowed to try again to repeal Obamacare, but they face a busy fall as Congress is approaching hard deadlines for raising the debt ceiling, funding the government, and reauthorizing the Children's Health Insurance Program.