Do you support free speech, individual liberties and protections for private property? You must be a liberal.

Did that last sentence cause you to do a double take? I’m not surprised.

I’ve been reading a new book, “The Closing of the Liberal Mind,” and it shows that much of what passes for liberalism today is, historically speaking, anything but. Moreover, many who call themselves conservative today would have been considered “liberal” if they lived in the time of the Founding Fathers.

This distinction isn’t mere semantics, however. The shift I’m describing goes right to the heart of the political and culture wars that rage around us today. Author Kim Holmes demonstrates why the authoritarian stance adopted by many liberals today — as exemplified by speech codes, trigger warnings, boycotts and shaming rituals — is in fact more accurately described as illiberalism.

You don’t have to go back to the days of the Enlightenment, John Locke and the French Revolution (as Mr. Holmes, a former U.S. assistant secretary of state, does in his commendably thorough book) to see this trend at work. Consider the following quote from a famous politician. See if you can guess who said it and when:

“I am sick and tired of people who say that if you debate and you disagree with this administration, somehow you’re not patriotic. We should stand up and say, ‘We are Americans, and we have a right to debate and disagree with any administration’.”

John McCain talking about President Obama? Nope. That’s Hillary Clinton. She said it in 2003, in reference to the George W. Bush administration.

But as they say, that was then, and this is now. Apparently it’s patriotic to express your opposition to President Bush, but if you utter a word against President Obama, you’re a lying, racist bigot. And if you deny it? Well, that’s just what we would expect a bigot like you to say.

And so most of today’s liberals (or “postmodern leftists,” to use Mr. Holmes’ preferred term) are not championing the right to offer opposing views, as their intellectual forebears would have done. They’re suppressing them.

This campaign to stamp out dissent takes many forms. We see it in Internal Revenue Service witch hunts against conservative groups. On college campuses with administrators meekly bowing to angry demands that politically incorrect speakers be banned. In the push from state attorneys general to investigate groups that question climate change.

Ask Lennart Bengtsson. In 2014, this well-respected Swedish meteorologist working in the United Kingdom joined a group called the Global Warming Policy Foundation, which questions some of the climate change community’s findings. Mr. Bengtsson did so simply because he was concerned that some of the computer-model predictions didn’t match up with actual scientific observations over time.

Big mistake. “Within a matter of days, he found himself in deep trouble,” Mr. Holmes writes. “As happened to other scientists who question any aspect of the global-warming ‘consensus,’ Bengtsson was hounded by colleagues to the point that he felt forced to resign from the think tank.”

Mr. Bengtsson cited concerns for his “health and safety,” saying the pressure made “normal work” virtually impossible, and warned: “It is a situation that reminds me about the time of McCarthy.”

That’s how far today’s “liberals” have fallen. They’ve become the very thing they once denounced.

And so we wind up in a culture that punishes young children severely for first-time, minor infractions under “zero tolerance” policies. One with so many thousands of federal laws that we prosecute adults for committing crimes that they didn’t even know were crimes.

“I disapprove of what you say, but I will defend to the death your right to say it.” Unless we all, liberal and conservative, find some way to recover this classically American outlook, the climate of intolerance will only grow more stifling. How far do we dare push it?