I'm a liberal Democrat. I support an expanded welfare state, stronger environmental regulation, and affirmative action in college admissions. I'm pro-choice, pro-Obamacare, and vehemently anti-Trump.

But I'm also an advocate for unbridled free speech, which makes me a "conservative" on many college campuses these days. Freedom of speech used to be a centerpiece of liberalism, while conservatives took up the banner for censorship. But in recent years, these roles have been reversed.

When you read about a speaker getting shouted down - or a campus newspaper getting confiscated - the censors are almost always on the Left. The latest example occurred earlier this month at Claremont-McKenna College, outside of Los Angeles, where students prevented conservative author Heather Mac Donald from giving a public address about her new book, The War on Cops. (Mac Donald delivered her remarks via livestream video, instead.)

The students' arguments - such as they were - were grimly predictable. By inviting Mac Donald to campus, the college administration allegedly gave its imprimatur to her views on race and policing. And those views made students - especially students of color - feel "unsafe."

How did two ideas that used to run in tandem - free speech and racial diversity - get pit against each other? Part of the answer lies in the remarkable growth of diversity itself. Between 1976 and 2012, the number of African American college students in the United States tripled. And women now receive 57 percent of undergraduate degrees, nearly double their proportion of 50 years ago.

Over the same span, more and more students reported mental-health problems. That reflected a new and welcome awareness of psychological illness, which lost some of its longstanding stigma.

Finally, new technologies inhibited in-person communication. More than half of community college students and a third of four-year college students agree with the statement, "I pretty much keep to myself socially." Even phone calls are avoided in favor of texting and social media, which give people more control over any interaction - and less anxiety about its outcome.

When you put these factors together, it's easy to see why there's less solicitude for free speech at colleges today. Arriving on campuses made up of diverse groups, students are warned that their comments and behavior could cause psychological distress to any of them. That's a pretty distressing prospect, in and of itself, so we shouldn't be surprised that many students would rather retreat to Facebook than risk offending someone to their face.

In a nationwide survey in 2015-2016, 71 percent of incoming freshmen agreed that "colleges should prohibit racist/sexist speech on campus." And 43 percent said that colleges should have the right to ban "extreme speakers," nearly double the proportion who agreed with that statement in 1971.

Who defines "extreme"? My students and I recently met with Mary Beth Tinker, who was 13 years old when she was suspended by her school in Des Moines, Iowa, for an extreme act: wearing a black armband protesting the Vietnam War. Her case wove its way up to the Supreme Court, which upheld her free-speech rights in the landmark Tinker v. Des Moines decision in 1969.

All of my students said they should be allowed to engage in antiwar demonstrations, of course, but they drew the line at racist or sexist speech that causes - yes - psychological injury. But Tinker wasn't having it. Surely, she said, parents whose children were fighting in Vietnam - or, especially, students whose parents had died there - were profoundly wounded by her very public act of protest. Yet that wasn't a good enough reason to silence her, or anybody.

Other students argued that free speech is really a matter of power, which has become another popular line on our campuses. In a society marred by racial inequality, the argument goes, speech is used by whites to oppress minorities. Hence white speech must be restrained, so that minorities can be protected.

But Tinker wasn't buying that, either. Historically, free speech has been a weapon - often, the only weapon - of the powerless, not the powerful. At the time her case began, Tinker reminded us, she was a child. And speech was all she had.

That's why every great champion of African American freedom in our history - including Frederick Douglass, W.E.B. Du Bois, and Martin Luther King Jr. - has also been a warrior for freedom of expression. "To suppress free speech is a double wrong," Douglass told a Boston audience in 1860, after a mob had broken up an anti-slavery meeting at the same location. "It is just as criminal to rob a man of his right to speak and hear as it would be to rob him of his money."

It's tempting to imagine that the mob at Claremont-McKenna was fighting against racism, so it was justified in squelching speech. But at the end of the day, a mob is still a mob. It's anathema to America's great liberal tradition, which relies on free speech to right our wrongs. Let's hope liberals on our campuses can rediscover it.

Jonathan Zimmerman teaches education and history at the University of Pennsylvania and is the author (with Emily Robertson) of "The Case for Contention: Teaching Controversial Issues in American Schools" (University of Chicago Press, April 2017). jlzimm@aol.com