Health care was a key factor in Democrats’ victory in the 2018 midterm elections, and it should be a big plus in 2020 as well. The shared Democratic position — that every legal resident should have access to affordable care, regardless of income or health status — is immensely popular. The de facto Republican position — that we should go back to a situation in which those whose jobs don’t come with health benefits, or who suffer from pre-existing medical conditions, can’t get insurance — is so unpopular that G.O.P. candidates consistently lie about their own proposals.

But right now, two of the major contenders for the Democratic presidential nomination, Joe Biden and Bernie Sanders, are having an ugly argument about health care that could hurt the party’s chances. There are real, important differences between the two men’s policy proposals, and it’s fine to point that out. What’s not fine is the name-calling and false assertions. Both men are behaving badly. And for their party’s sake, and their country’s, they need to stop it.

Let’s back up. There are, broadly speaking, two ways a country can try to achieve universal health insurance. One is single-payer: The government simply pays the bills. The other retains a role for private insurance but relies on a combination of regulations and subsidies to ensure that everyone gets covered.

We don’t have to speculate about how these systems would work in practice, because every advanced country except the U.S. has some form of universal coverage. Some, like Canada and Britain, use single-payer (in Britain the government also operates the hospitals and pays the doctors). Others, like Switzerland and the Netherlands, have a large role for private insurers.