When it comes to race, Mr. Trump plays with fire like no other president in a century. While others who occupied the White House at times skirted close to or even over the line, finding ways to appeal to the resentments of white Americans with subtle and not-so-subtle appeals, none of them in modern times fanned the flames as overtly, relentlessly and even eagerly as Mr. Trump.

His attack on the Democratic congresswomen came on the same day his administration was threatening mass roundups of immigrants living in the country illegally. And it came just days after he hosted some of the most incendiary right-wing voices on the internet at the White House and vowed to find another way to count citizens separately from noncitizens despite a Supreme Court ruling that blocked him from adding a question to the once-a-decade census.

His assumption that the House Democrats must have been born in another country — or that they did not belong here if they were — fits an us-against-them political strategy that has been at the heart of Mr. Trump’s presidency from the start. Heading into next year’s election, he appears to be drawing a deep line between the white, native-born America of his memory and the ethnically diverse, increasingly foreign-born country he is presiding over, challenging voters in 2020 to declare which side of that line they are on.