“He’s dehumanized or sought to dehumanize those who do not look like or pray like the majority here in this country,” Mr. O’Rourke said.

Senator Bernie Sanders of Vermont, another leading candidate for the Democratic nomination, also believes Mr. Trump is a white supremacist. Mr. Sanders was asked on CNN on Sunday if he agreed that the president was “a white supremacist or a white nationalist,” and Mr. Sanders replied, “I do.” A senior campaign official confirmed on Thursday that Mr. Sanders believed Mr. Trump was both.

Mr. Trump has a long history of using race for his own gain, and his time in the White House has been no exception.

After pushing the “birther” lie about President Barack Obama, Mr. Trump began his campaign for the presidency by disparaging Mexican immigrants as rapists and criminals. As president, he sought to bar people from predominantly Muslim countries from entering the United States; said there were “very fine people on both sides” of a white supremacist rally in Charlottesville, Va.; and used an obscenity to describe African nations.

He has warned of an “invasion” of migrants at the southern border. And last month, he said that four congresswomen of color should “go back” to the countries they came from; all four of the women are American citizens and only one was born outside the United States.