[Mr. Trump lashed out again Monday, accusing the women of themselves using “racist hatred.”]

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.

“In many ways, this is the most insidious kind of racial demagoguery,” said Douglas A. Blackmon, the author of “Slavery by Another Name,” a Pulitzer Prize-winning history of racial servitude in America between the Civil War and World War II. “The president has moved beyond invoking the obvious racial slanders of 50 years ago — clichés like black neighborhoods ‘on fire’ — and is now invoking the white supremacist mentality of the early 1900s, when anyone who looked ‘not white’ could be labeled as unwelcome in America.”

Mr. Trump ritually denies any racial animus or motivations. His fight against illegal immigration, he says, is only about securing the border and protecting the country. He regularly boasts that unemployment among Hispanics and African-Americans has hit record lows. Last week he thanked Robert L. Johnson, the founder of Black Entertainment Television, for crediting his stewardship of the economy.

“I am the least racist person you have ever met,” he has said more than once.

But he does not go out of his way to avoid looking like he is, and his string of Twitter posts on Sunday left his own advisers unable or unwilling to defend him. None of six spokespeople for the White House or his campaign initially responded to requests for comment.

One of the only administration officials who was already booked for the Sunday talk shows, Mark Morgan, the acting commissioner of Customs and Border Protection, made clear he wanted no part of it. “You’re going to have to ask the president what he means by those specific tweets,” he said on “Face the Nation” on CBS.