But the reasons why the single-payer push helps Trump and the GOP go well beyond the health-care debate itself. This emerging litmus test is symptomatic of a larger problem: The Democratic party is increasingly the wholly owned subsidiary of the progressive base, and the progressive base has become too radical for widespread electoral success.

Here, the results speak for themselves. President Obama’s personal popularity obscured the unpopularity of his policies, and the Democrats have steadily collapsed as a national political party. Its appeal is concentrated in densely populated coastal urban enclaves, and it keeps generating political ideas that appeal primarily to this urban base — and no one else.

That same dense urban concentration yields the groupthink that very quickly becomes angry intolerance. Debates quickly move to the extreme, and name-calling substitutes for argument. Don’t think a man can get pregnant? You’re a bigot. Do you believe that students accused of sexual assault should enjoy the right to counsel or the right to cross-examine witnesses? You’re a rape apologist. Are you wary of government-run health care, and do you believe the free market can deliver better care? Then you’re killing people.