That is the danger facing the Democrats, and particularly their quasi-front-runner Elizabeth Warren, with the issue of “Medicare for all.” Single-payer health care is, in certain ways, the liberal-activist equivalent of the conservative dream of a flat tax. It’s an idea of some merit if you’re designing a system from scratch and it polls O.K. if you don’t tell people about the trade-offs. But it tends to run into trouble quickly on the state level — with Vermont’s stillborn single-payer experience mirroring the flat-tax experiments of states like Kansas. And it has enough political vulnerabilities, in terms of costs and disruption both, that no sane Democrat should want it as the centerpiece of their national campaign.

Warren, who is definitely sane, clearly doesn’t want to make it her centerpiece; you can tell that she’d like to run on her promise to tax the wealthy to pay for free child care and college, with a dose of anti-corruption and trustbusting on the side. These ideas have their own difficulties, but they’re popular and responsive to the voters, and a good foil for Donald Trump’s record of corporate tax cuts and not much else.

But being a progressive candidate in a leftward-marching party required her to sign on to Medicare for all, and being the “I’ve got a plan for that” candidate in a party that still fetishizes wonkery required her to roll out a big, multi-trillion-dollar proposal on Friday. (Kamala Harris, the Tim Pawlenty of 2020, attempted Medicare-for-all evasiveness and fell off the map.) So now if Warren wins the nomination she’s going to drag a multi-trillion dollar renovation of the American health care system into the fall campaign — even though everyone understands that the renovation won’t happen, even though Warren herself would rather talk about other policies that poll better. An ideological party is a harsh mistress.

Of course, Team Warren and its many adjuncts in the press will argue that one, this is the primary campaign and there’s time to pivot toward the center, and two, that Warren’s plan is super-clever in its avoidance of overt middle class taxation.