The clear loser of the Democratic primary is Medicare for All.

First, the policy demonstrated the unreliability of Kamala Harris out of the gate, when she endorsed it before quickly backing off. Now, it has blunted the momentum of Elizabeth Warren, made a mockery of her claim to be an uber-wonk and shredded her implicit appeal to Bernie Sanders supporters as an equally committed left-winger without the baggage.

Under pressure for weeks for details related to her version of the proposal, Warren has now backed all the way down to promising to pass Medicare for All by the end of the third year of her presidency.

This is an implicit concession that she won’t do it at all.

No presidential candidate ever pledges to do something important to him in Year Three. That’s when, if history is any guide, a president has suffered a midterm drubbing and lost all legislative momentum. Warren wants us to believe that this would be the ­opportune time for her to pass perhaps the most sweepingly ­intrusive government measure in American history.

Besides, how does Warren ­expect this midterm to go if it is fought, as it inevitably would be, on a proposal so far-reaching and radioactive that she didn’t dare ­offer it in the initial phase of her presidency?

Like almost all the Democrats early in the race, Warren’s fundamental mistake was to believe she had to chase Sanders around the track, which inevitably involved backing his signature health-care proposal. But it became immediately evident that it’s one thing to promise to eliminate all private health insurance if you are a self-declared socialist; it’s quite another if you imagine yourself anything short of that.

As soon as another erstwhile Bernie-band-wagoner, Harris, ­uttered out loud that she would end private health insurance, it created a controversy that she was clearly uncomfortable with. As a way to wiggle out of it, she came up with her own plan.

Warren lasted longer. Her undoing was that her resolute unwillingness to say that she’d raise middle-class taxes to pay for the program undermined her self-image as a woman with a “plan for that.”

She had to jerry-rig a ­financing program built on such outlandishly rosy assumptions about costs and revenues that even her journalistic cheerleaders have been skeptical. As she continued to take fire, Warren ­announced her “transition” plan, effectively signaling the program is not a first-term priority.

In so doing, she has managed to bring on herself the worst of both worlds. Democratic purists will be disappointed in her, and Sanders voters feel confirmed in any doubts they already had about her commitment. Meanwhile, she still formally favors a plan to eliminate every private health-insurance plan in America, opening her up to justifiably savage Republican attacks should she win the Democratic nomination.

It should have been foreseeable that proposing a ruinously expensive, enormously coercive health program would present political problems. Even Democratic primary voters aren’t fully sold on a Medicare for All plan that eliminates all private insurance.

Besides, it defies the approach that has worked for the party for decades, which is rejecting politically perilous wholesale changes to health care in favor of salami-slice increases in government ­involvement.

Sanders has gotten away with it because socialism is his brand and conviction. He hand waves away questions on the specifics — what do they matter, when the revolution will make all things possible?

By contrast, Warren let the critics get into her head, just as she did over her purported Native American heritage, and stumbled into a messy, self-destructive ­response, just as she did with her DNA test earlier in the year. Democrats have to be wondering, over and above her struggles with the Medicare for All, if this is really who they want to send up against the endlessly combative and needling President Trump next year.

Twitter: @RichLowry