Who is a racist? Definitions vary in red and blue America.

If one word has dominated the U.S. news cycle lately, it’s probably “racist.” But the shouting match over presidential tweets is surfacing a divide over the definition of that incendiary word.

Hours before President Donald Trump held a rally in Greenville, North Carolina, on Wednesday night, nobody waiting in the heat and anticipation of the parking lot outside an East Carolina University auditorium said he was a racist. Nobody asked by a reporter thought his tweet telling four female members of Congress to “go back” to the “places from which...