The opportunity for Democrats, however small, is fairly clear here: It’s reasonable to assume higher turnout would draw from a pool of voters who are relatively likely to disapprove of the president.

The opportunity for Republicans is somewhat more subtle, but clear as well. The voters who turned out in 2016, but stayed home in 2018, were relatively favorable to Mr. Trump, and they’re presumably more likely to join the electorate than those who turned out in neither election. In a high-turnout election, these Trump supporters could turn out at a higher rate than the more Democratic group of voters who didn’t vote in either election, potentially shifting the electorate toward the president.

Those who aren’t registered but still might vote

A high-turnout election would draw from another group of voters: those who aren’t yet registered.

These voters are hard to measure. They are underrepresented in public opinion surveys, and there’s reason to wonder whether those who do take surveys are representative of those who don’t. They are also less likely to hold opinions on current events, including on the president. (For ease of comparison, those without an opinion of the president have been excluded from measures of the president’s approval rating.)

With those caveats in mind, the president’s approval rating among nonregistered voters stood at just 37 percent in an Upshot compilation of 12 surveys, conducted between December 2017 and September 2018, by the Pew Research Center and the Kaiser Family Foundation. Mr. Trump’s approval rating was at 43 percent among registered voters in the same collection of surveys.

The data includes over 14,000 registered voters and nearly 3,200 voters who aren’t registered, allowing for a fairly detailed analysis and comparison of the groups.

The president’s weakness among nonregistered voters is consistent with a long record of polling showing Democrats fare better among all adults than among registered voters, including in today’s FiveThirtyEight averages.

The potential for Democrats is obvious. But in general, these figures — and other polls comparing the adult and registered voter populations — exaggerate the opportunity available to Democrats because they include noncitizens, who aren’t eligible to vote.