Speaking in Charleston at Emanuel African Methodist Episcopal Church, where a white supremacist gunman killed nine black worshipers in 2015, Senator Cory Booker of New Jersey also blamed Mr. Trump for encouraging hatred. The weekend’s violence, he said, was “sowed by those who spoke the same words the El Paso murderer did, warning of an ‘invasion,’” a word Mr. Trump has used to describe migrants approaching the Southern border.

And Senator Elizabeth Warren and former Representative Beto O’Rourke both said they believe Mr. Trump was a white supremacist.

After the president’s visit to El Paso on Wednesday, Mr. O’Rourke also suggested that Mr. Trump bore responsibility for the mass shooting there on Saturday by a white supremacist gunman who killed 22 people. Referring to immigrants, Mexicans and others in the community there, Mr. O’Rourke said, “To have been so regularly attacked and vilified and demonized by this president, for him to have created the conditions that made an attack like this possible and ultimately likely — it’s very insulting for us that he was here.”

Mr. Trump has emphatically denied that he is racist, and on Wednesday, he dismissed reporters’ questions about the role of his rhetoric in dividing the country, saying his language “brings people together.”

The extraordinary focus this week on white nationalism, gun violence and domestic terror appeared to reframe a chaotic presidential campaign as a searing moral debate about the racial history and cultural destiny of the United States. Mr. Trump, who rose to power railing against the country’s changing ethnic and cultural texture, contends that Democrats should be punished for opposing his immigration policies and rejecting the values of the rural white people who make up his base. Democrats, meanwhile, are now arguing in the most explicit terms yet that white supremacists are receiving aid and comfort from the president.