Bret Weinstein is a biology professor at Evergreen State College in Olympia, Wash., who supported Bernie Sanders, admiringly retweets Glenn Greenwald and was an outspoken supporter of the Occupy Wall Street movement.

You could be forgiven for thinking that Mr. Weinstein, who identifies himself as “deeply progressive,” is just the kind of teacher that students at one of the most left-wing colleges in the country would admire. Instead, he has become a victim of an increasingly widespread campaign by leftist students against anyone who dares challenge ideological orthodoxy on campus.

This professor’s crime? He had the gall to challenge a day of racial segregation.

A bit of background: The “Day of Absence” is an Evergreen tradition that stretches back to the 1970s. As Mr. Weinstein explained on Wednesday in The Wall Street Journal, “in previous years students and faculty of color organized a day on which they met off campus — a symbolic act based on the Douglas Turner Ward play in which all the black residents of a Southern town fail to show up one morning.” This year, the script was flipped: “White students, staff and faculty will be invited to leave campus for the day’s activities,” reported the student newspaper on the change. The decision was made after students of color “voiced concern over feeling as if they are unwelcome on campus, following the 2016 election.”

Mr. Weinstein thought this was wrong. The biology professor said as much in a letter to Rashida Love, the school’s Director of First Peoples Multicultural Advising Services. “There is a huge difference between a group or coalition deciding to voluntarily absent themselves from a shared space in order to highlight their vital and under-appreciated roles,” he wrote, “and a group or coalition encouraging another group to go away.” The first instance, he argued, “is a forceful call to consciousness.” The second “is a show of force, and an act of oppression in and of itself.” In other words, what purported to be a request for white students and professors to leave campus was something more than that. It was an act of moral bullying — to stay on campus as a white person would mean to be tarred as a racist.