For now, the mostly white working-class Rust Belt states, decisive in the 2016 election, remain at the center of the electoral map, based on our estimates. The Democrats have few obviously promising alternative paths to win without these battleground states. The president’s approval ratings remain higher in the Sun Belt battlegrounds than in the Rust Belt, despite Democratic hopes of a breakthrough.

The president’s views on immigration and trade play relatively well in the Northern battlegrounds, including among the pivotal Obama-Trump voters.

There are signs that some of these voters have soured on his presidency, based on recent polling. There is also reason to think that white working-class voters who supported Mr. Trump were relatively likely to stay home in last November’s midterm elections.

A strategy rooted in racial polarization could at once energize parts of the president’s base and rebuild support among wavering white working-class voters. Many of these voters backed Mr. Trump in the first place in part because of his views on hot-button issues, including on immigration and race.