While Mr. Trump was blunt in his attacks on immigrants, he has also begun to highlight issues involving African-Americans, the police and crime, using language that to some party officials evokes earlier appeals to white prejudice and anxiety. Mr. Trump and other prominent Republicans have also begun to directly criticize the Black Lives Matter movement, which seeks to call attention to mistreatment of African-Americans by police officers.

Amid an increase in murders in a number of cities and the high-profile killing of police officers, Mr. Trump has been infusing his speeches with calls for “law and order.” Echoing former President Richard M. Nixon, he has said that a “silent majority” will join him in taking back the country, and he has said he will rid heavily black Ferguson, Mo., Baltimore and Chicago of gangs and “tough dudes.”

“That’s not a dog whistle; that’s a dog siren,” Rick Wilson, a Florida-based Republican strategist, said of Mr. Trump’s references to cities, gangs and policing. “When he first started saying ‘silent majority,’ I didn’t think he understood the historical antecedents, but now I believe they very much do.”…

“This kind of politics of anger seems to be taking us back to some ugly moments in American history,” said Russell Moore, a senior official with the Southern Baptist Convention. “It’s a regrettable and dangerous ploy that I don’t think churchgoing evangelicals are going to fall for.”