The Palestinian activist Omar Barghouti, one of the founders of the boycott, divestment and sanctions movement, was supposed to be on a speaking tour of the United States this week, with stops at N.Y.U.’s Washington campus and at Harvard. He was going to attend his daughter’s wedding in Texas. I had plans to interview him for “The Argument,” the debate podcast that I co-host, about B.D.S., the controversial campaign to make Israel pay an economic and cultural price for its treatment of the Palestinians.

Yet when Barghouti, a permanent resident of Israel, showed up for his flight from Israel’s Ben Gurion International Airport last week, he was informed that the United States was denying him entry. When I spoke to him on Sunday, he still didn’t know exactly why the country where he went to college and lived for many years wasn’t letting him in, but he assumed it was because of his political views. If that’s the case, Barghouti said, it was the first time someone has been barred from America for B.D.S. advocacy. He has proceeded with his public events, but he’s been appearing at them via Skype.

In recent years, the American right has presented itself as a champion of free expression. Conservatives are constantly bemoaning a censorious campus climate that stigmatizes their ideas; last month, Donald Trump signed an executive order on campus free speech, decrying those who would keep Americans from “challenging rigid far-left ideology.” The president said, “People who are confident in their beliefs do not censor others.”

If that last line is true — and, uncharacteristically for Trump, I think it is — it says something about the insecurity of Israel’s defenders. There have indeed been illiberal attempts to silence conservative voices on college campuses, but they pale beside the assault on pro-Palestinian speech, particularly speech calling for an economic boycott of Israel. Around two dozen states have laws and regulations denouncing, and in many cases penalizing, B.D.S. activities, and the Senate recently passed a bill supporting such measures. According to the American Association of University Professors, some public universities in states with such laws require speakers and other contractors to “sign a statement pledging that they do not now, nor will they in the future, endorse B.D.S.” It’s hard to think of comparable speech restrictions on any other subject.