In every presidential election for the past 50 years, a majority of white voters have voted against the Democratic nominee, and the overwhelming majority of people of color have sided with the Democrats.

But there is a determined and consistent core of whites who always vote Democratic. Since the advent of exit polling of racial groups in 1976, no Democrat has received less than 34 percent of the white vote (that was Walter Mondale in 1984, losing 49 states in a landslide to Ronald Reagan). The historical average of white support for Democrats is almost 40 percent, and Hillary Clinton, up against Mr. Trump’s thinly disguised call to Make America White Again, garnered the support of 37 percent of white voters.

What we learned in the 2016 election is that 37 percent of the white vote is enough to win the popular vote by nearly three million people. Obviously something went wrong in three critical states — Michigan, Wisconsin and Pennsylvania — where Mr. Trump prevailed by nearly 80,000 votes, tipping the Electoral College in his favor. But many progressives are drawing the wrong conclusions about what happened in those states.

It’s true that the requisite amount of white support for a Democrat to win in those three states is approximately 3 percent higher than the number necessary to win the national popular vote. The exit poll data from 2016 shows two major realities that are important for current political analysis and strategy. Mrs. Clinton came exceedingly close to winning those states. Had she secured just 0.5 percent more of the white vote, she would be president.

But perhaps even more important, Mrs. Clinton’s diminished white support was not primarily a result of Democratic defections to Mr. Trump (the now near-mythical “Obama-Trump voter”). The increase in white support for the third- and fourth-party candidates — from 2012 to 2016 — was greater than the increase for Mr. Trump. In fact, in Wisconsin, he got fewer votes than Mitt Romney received four years earlier, disproving the notion that waves of disaffected Democratic voters swelled the Republican ranks. If everything else holds steady in 2020, and Democrats win back just the Obama-Jill Stein voters, they will take Michigan and Wisconsin.