The concept of “Medicare for All” has majority support in 42 states, according to the small liberal policy shop Data for Progress. Using numbers from the Kaiser Family Foundation, which tracks public opinion about health-care policy, a handful of researchers found that the concept of universal Medicare was popular in all but two states (Montana and West Virginia) where Democratic senators are seeking reelection this year.

“We find that there are a number of states, such as New York, Illinois, Rhode Island and California, where there is majority support for Medicare for All but Democratic senators who have not signed on to S1804,” said Data for Progress’s Sean McElwee, giving the number for the Medicare legislation introduced by Sen. Bernie Sanders (I-Vt.).

There were 1,202 respondents in the survey Data for Progress used to calculate Medicare for All’s support, and the rest of the number crunching focused on voter engagement on the issue. Support for the policy was highest among voters ages 18 to 29 (above 60 percent), black voters (above 75 percent) and voters making less than $20,000 (above 65 percent); it was lowest among white voters and voters making more than $100,000.

Pollsters caution that the popularity of the concept — Medicare being expanded into a basic universal coverage program, paid for through higher tax rates — has not been tested in a competitive election.

With support growing for universal health coverage, just what does "single-payer" mean? Here's the breakdown. (Jenny Starrs, Danielle Kunitz/The Washington Post)

“From a research perspective, there really is no such thing today as fully informed public opinion verdict on single-payer insurance or Medicare for All,” said Democratic pollster Geoff Garin. “More than any other issue, voters litigate health-care policy on a personal basis and really sweat the details. The Republicans learned to their sorrow how much the details matter when support for the idea of repeal and replace collapsed after voters saw the reality of repeal and replace. Support for Medicare for All may be more durable, but there is a lot to play out before we know.”

Since the apparent victory of Conor Lamb in a special election for Pennsylvania’s 18th Congressional District, some Democrats have speculated that his center-left policy agenda — falling short of endorsing universal Medicare and the $15 minimum wage — is easier to sell in swing seats than the full progressive agenda. In an interview, Sanders allowed that candidates like Lamb could tailor their campaigns to the district but suggested it would be hard to motivate the electorate without universal Medicare.

Look, America is a very, very big country,” Sanders said. “People running in Northern California are not necessarily going to be the same as people running in Mississippi or western Pennsylvania. The way to increase the voter turnout, and the way for Democrats to win, is to run on the progressive agenda, because that’s what Americans support. Medicare for All, the $15 minimum wage, free college — that’s good policy and good politics.”

Jeff Stein contributed reporting.