As House Democrats sit down to draft their vision of governance in the coming weeks, lawmakers find themselves badly divided.

Centrists from swing districts, with the tacit support of Speaker Nancy Pelosi, favor incremental moves to shore up the Affordable Care Act and to lower the out-of-pocket costs of prescription drugs and medical care. They are pushing a variety of measures, such as shutting down cheap, short-term insurance plans that do not cover pre-existing medical conditions and allowing people to buy into Medicare at age 50 or 55.

But they are butting up against an aggressive and expanding group of more than 100 outspoken Democrats — as well as at least four of the party’s presidential candidates — who want to upend the whole system with a single government insurance plan for all Americans — the old concept of single payer, now called Medicare for all.