What does Ryan mean when he says 20 million Americans will lose their health insurance under Obamacare?

He is referring to a recent CBO study that gave several scenarios for what could happen to employer-based coverage once the law was implemented. The most positive scenario has 3 million people being added to employer coverage, while “on balance, the number of people obtaining coverage through their employer would be about 3 million lower in 2019 under the legislation than under prior law,” the CBO concludes.

The worst-case scenario was 20 million people, which is where Ryan got his number. It’s worth noting that the baseline scenario — 3 million fewer people — represents just 2 percent of the people who now get insurance through their employers.

The CBO cautions that there is a “tremendous amount of uncertainty” about how employers and employees will respond to the legislation. “One piece of evidence that may be relevant is the experience in Massachusetts, where employment-based health insurance coverage appeared to increase after that state’s reforms,” the CBO noted. Mitt Romney, as governor, ushered in health-care legislation that served as a model for Obama’s health plan.