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

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 they came” was racist at all.

The tweet was vague and didn’t refer directly to race, many said. Besides, “racist” is thrown around too much, in their view. Democrats use it...