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.