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.

Reasonable people can debate whether or not social experiments like a Day of Absence are enlightening. Perhaps there’s a case to be made that a white-free day could be a useful way to highlight the lack of racial diversity, particularly at a proudly progressive school like Evergreen. Yet reasonable debate has made itself absent at Evergreen.

For expressing his view, Mr. Weinstein was confronted outside his classroom last week by a group of some 50 students insisting he was a racist. The video of that exchange — “You’re supporting white supremacy” is one of the more milquetoast quotes — must be seen to be believed. It will make anyone who believes in the liberalizing promise of higher education quickly lose heart. When a calm Mr. Weinstein tries to explain that his only agenda is “the truth,” the students chortle.

Following the protest, college police, ordered by Evergreen’s president to stand down, told Mr. Weinstein they couldn’t guarantee his safety on campus. In the end, Mr. Weinstein held his biology class in a public park. Meantime, photographs and names of his students were circulated online. “Fire Bret” graffiti showed up on campus buildings. What was that about safe spaces?

Watching the way George Bridges, the president of Evergreen, has handled this situation put me in mind of a line from Allan Bloom’s book “The Closing of the American Mind.” Mr. Bloom was writing about administrators’ reaction to student radicals in the 1960s, but he might as well be writing about Evergreen: “A few students discovered that pompous teachers who catechized them about academic freedom could, with a little shove, be made into dancing bears.”

At a town hall meeting, Mr. Bridges described the protestors as “courageous” and expressed his gratitude for “this catalyst to expedite the work to which we are jointly committed.” Of course, there was also pablum about how “free speech must be fostered and encouraged.” But if that’s what Mr. Bridges really believes, why isn’t he doing everything in his power to protect a professor who exercised it and condemn the mob that tried to stifle him?

The Weinstein saga is just the latest installment in a series of similar instances of illiberalism on American campuses. In March, a planned speech by Charles Murray at Middlebury ended with the political scientists escorted off campus by police and his interviewer, Professor Allison Stanger, in a neck brace. In April, a speech at Claremont McKenna by the conservative writer Heather Mac Donald had to be livestreamed when protestors blocked access to the auditorium.

Shutting down conservatives has become de rigueur. But now anti-free-speech activists are increasingly turning their ire on free-thinking progressives. Liberals shouldn’t cede the responsibility to defend free speech on college campuses to conservatives. After all, without free speech, what’s liberalism about?