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A couple of weeks ago, one of the country’s most respected health care pollsters — Kaiser Family Foundation — conducted a survey on “Medicare for All.” And the top-line results looked great for advocates of the idea, like Bernie Sanders and Kamala Harris.

Some 56 percent of respondents said they favored “a national plan called Medicare for All in which all Americans would get their insurance through a single government plan.” A large majority of Democrats backed the idea. Almost a quarter of Republicans did, too.

The poll’s details, however, were a lot of less positive about Medicare for All. In fact, they showed why single-payer health care may turn out to be one of the few problematic issues for Democrats heading into 2020 — if the party isn’t careful. Harris has highlighted the tensions this week, saying on Monday night that she supported the most aggressive version of Medicare for All before moderating her position, via aides, late yesterday.