“Within white supremacy, there’s this idea of dispossession, that you are being dispossessed of your goods, your opportunities, perhaps your land,” Ms. Beirich added. “And they point to South Africa to say to fellow white supremacists or people who they see as potential recruits that this is what could happen to us.”

Mike Peinovich, a white supremacist whose podcast is called “The Daily Shoah,” called the president’s tweet about South Africa “very big.” He said on Twitter: “It may seem like a small thing, but this is how we slowly chip away at the all consuming anti-white discourse. Let’s hope this is followed with action.”

Kallie Kriel, the chief executive of AfriForum, South Africa’s main advocacy group for Afrikaners, said that Mr. Trump’s comment had been a “huge step forward” for his organization. (Afrikaners are the white settlers of South Africa, usually of Dutch or Huguenot ancestries. Boer is sometimes used as a term for Afrikaners, typically in a specific political context.)

On a tour of the United States this year, Mr. Kriel met with representatives of conservative think tanks in Washington, including the Cato Institute and the Heritage Foundation, as well as an aide to Senator Ted Cruz, Republican of Texas, and officials at the United States Agency for International Development.

During the trip, Ernst Roets, the group’s deputy chief executive, appeared on “Tucker Carlson Tonight” on Fox — a staple of Mr. Trump’s cable news diet. In the studio, he and Mr. Kriel encountered John R. Bolton, Mr. Trump’s national security adviser, and snapped a picture with him that Mr. Kriel posted on Twitter, marveling at their “luck.”

He said they had given Mr. Bolton a copy of “Kill the Boer,” Mr. Roets’s book that claims the government has been complicit in killing white farmers. It also argues, according to the official website, that “a looming process of ethnic cleansing should be regarded as a serious threat and something to be prevented.”

On Thursday, the White House did not respond to requests for comment on what had prompted Mr. Trump to tweet about the issue.