The increase in his support since 2016, and the possibility that it continues to move higher, does not necessarily make him a favorite to win re-election. His job approval ratings remain well beneath 50 percent, and have never eclipsed it. But the rise has some important implications in how to view his re-election prospects.

One common view of the 2020 election, for instance, takes 2016 as a starting point. It notes that Democrats fell just short of victory, and that therefore any number of changes — a better candidate, higher black turnout, and so on — would be enough to win the election in 2020. This way of thinking assumes that the president’s support would remain unchanged — that he could do little to match incremental increases in Democratic turnout or support, compared with 2016.

But it is not 2016 anymore. Millions of Americans who did not like the president in 2016 now say they do. Over all, his personal favorability rating has increased by about 10 percentage points among registered voters since Election Day 2016, to 44 percent from 34 percent, according to Upshot estimates.

Some of these voters probably voted for Mr. Trump in 2016, even though they didn’t like him at the time. But some probably did not vote for him: Republicans with an unfavorable opinion of Mr. Trump were more than twice as likely to stay home on Election Day as those with a favorable view, according to New York Times/Siena surveys of North Carolina, Florida and Pennsylvania in 2016.

It seems likely that a substantial number of these voters now have a favorable view of the president: Over all, 28 percent of Republican-leaning voters with an unfavorable view of Mr. Trump in 2016 had a favorable view of him by 2018, according to data from the Voter Study Group. The aggregate national data suggests that Mr. Trump has gained more support than that — if not from Republicans then perhaps from some number of independents or former Democrats.