A few months ago, Sen. Kamala Harris raised eyebrows when she nonchalantly proclaimed her desire to abolish private health insurance: “Let’s move on.” Today, Ms. Harris’s quip seems quaint. The latest liberal policy idea would effectively end all private health care for many Americans.

The proposal, the Medicare for America Act, first appeared as a 2018 paper by the Center for American Progress. It was a plan to expand government-run health care. It’s been called “the Democratic establishment’s alternative” to Sen. Bernie Sanders’s single-payer scheme. In March, Democratic presidential hopeful Beto O’Rourke endorsed Medicare for America in lieu of the Sanders plan.

CNN declared that Mr. O’Rourke’s endorsement of Medicare for America demonstrates his “moderate path,” but the bill is anything but moderate. When Rep. Rosa DeLauro reintroduced Medicare for America legislation on May 1, she included a new, radical provision. The revised bill prohibits any medical provider “from entering into a private contract with an individual enrolled under Medicare for America for any item or service coverable under Medicare for America.” Essentially, this would bar program enrollees from paying for health care using their own money.

Liberals might claim this prohibition is more innocuous than it sounds, because Americans can still use private insurance under Medicare for America in some circumstances. But the legislation squeezes out the private insurance market in short order.

For starters, the law would automatically enroll babies in the new government program at birth. The Center for American Progress’s original paper admitted that the auto-enrollment language would ensure the government-run plan “would continue to grow in enrollment over time.” The bill would permit people to opt out of the government program only if they have “qualified health coverage” from an employer. And even employer-provided health insurance would soon disappear.