A two-tiered system is by no means destined to happen under Medicare for All—more on that later. But some countries that transitioned to single-payer health care have, indeed, found their health systems bifurcating. In 1988, Brazil promised free public health care to every citizen, and it has delivered, in the form of the Sistema Único de Saúde, or SUS. As a result, Brazilians got healthier and lived longer, and they have very few catastrophic health expenditures.

Still, the system looks more like “a safety net with holes,” as one doctor put it to me a few years ago. The public SUS is rife with shortages and long waits. “You have to take four buses to get to the ER,” one physician told me. “Your stomach could explode before you get there.” Instead, wealthy Brazilians go to private doctors using private insurance plans.

A two-tiered system doesn’t have to be a disaster, of course—and it’s not necessarily worse than what we have. (Indeed, just a few years ago, leading health-care economists were arguing for a two-tiered health system in the United States.) In the U.K., some providers take private insurance and cash, while government-salaried National Health Service doctors take all patients for free. Yet the U.K. is still considered to have a better-performing health-care system than the United States. Even one of Senator Bernie Sanders’s favorite countries, Denmark, has a two-tiered health system.

Instead, whether the two-tiered system actually happens, or affects Americans negatively, hinges on the ultimate shape that a potential Democratic administration’s health plan takes. We just don’t know enough yet about what the prospective Democratic victor would be able to get passed through Congress. Would the public system be adequately funded? Then it would work better than the shoddy Brazilian SUS. Would doctors be required to accept Medicare? If so, doctors wouldn’t be able to bill patients privately, period. Perhaps most important, would Medicare-reimbursement rates be higher than they are today? That would make doctors and hospitals, and maybe even voters, more likely to accept the plan happily.