Joe Biden, Pete Buttigieg and Mike Bloomberg claim they’re proposing a moderate, less disruptive approach to health-care reform when they advocate a public option—a government policy offered as an alternative to private health insurance—in lieu of Medicare for All. Don’t believe it. My research finds that such proposals would increase the federal deficit dramatically and destabilize the market for private health insurance, threatening health-care quality and choice.

While estimates by the Congressional Budget Office and other...