Sen. Bernie Sanders’s (I-VT) is introducing a single-payer health bill on Wednesday that would enroll every American resident on Medicare within four years, whether they want to be on it or not.

Doing so would provide health insurance to the 28 million Americans who currently lack it, while bringing relief to the tens of millions of lower- and middle-class Americans who say they still struggle to pay their medical bills. The bill would require a tax hike, likely in the trillions of dollars, and force upward of 150 million people to switch from their current employer-based insurer to the single government-run plan. The government would also have to figure out how to run and administer insurance for 330 million people.

The core idea of Sanders’s single-payer bill is that the government can and should pay for basically all medical treatments of the whole population. It treats how to pay for doing so largely as an afterthought, and promises to work backward from the premise of universal care. A shocking number of Senate Democrats have embraced the bill this week, with about a third signing on — an indicator to the bill’s supporters that the party may be finally willing to stand for the right of the poor to have health care, but a sign to the bill’s critics that Democrats are moving too far to the big-government left.

Sanders’s Medicare-for-all Act of 2017 begins by transforming Medicare so that its enrollees do not pay any money for virtually limitless free health insurance — with the exception of cosmetic care, like plastic surgery. (As currently operated, Medicare enrollees have to pay significant costs in premiums and deductibles to access their care.) Then the bill would drop the Medicare eligibility age — from 65 to 55 in the first year, and then gradually over four years until everyone is covered.

Sanders has not explained how he’d pay for the plan — which puts the government on the hook for unlimited health care for 330 million Americans — but is promising to release possible tax models for doing so in the coming weeks.

The bill would not force patients to have to switch their doctors or hospitals, just their insurers. Every American would receive a “Universal Medicare” card with their insurance information, and all providers qualify to participate under Medicare’s existing rules.

In an interview, Sanders pledged that all middle-class families would net savings because their health care spending would go down more than their taxes would increase. Some critics have been skeptical when Sanders made similar claims in the past.