Cary Nelson is a English professor at the University of Illinois at Urbana-Champaign and an affiliated professor at the University of Haifa. David Greenberg is a history professor at Rutgers University. They are members of the Alliance for Academic Freedom.

Since 2014, there has been a disturbing surge in the number of invited campus speakers being repeatedly interrupted or actually prevented from delivering a public lecture. A startling share of these silencing efforts have been directed at Israelis or other speakers sympathetic to Israel who have run afoul of the growing anti-Israel movement on campuses.

Behind this spike is an idea called “anti-normalization.” This concept, which anti-Israel organizations began vigorously promoting two years ago, holds that any activities that might “normalize” relations between Israelis and Palestinians — from children’s soccer leagues to collaborative environmental projects to university panel discussions with both sides represented — should be summarily rejected because they treat both parties as having legitimate grievances and aspirations. Joint projects are to be shunned unless they begin with the premise that Israel is the guilty party.

Shouting down speakers — including defenders of Israel — didn’t start with the adoption of anti-normalization. But in the past such episodes were regarded as exceptional and scandalous violations of academic freedom. In one of the first of these episodes, in 2010, Michael Oren, a distinguished historian serving as Israel’s ambassador to the United States, tried to give a presentation at the University of California at Irvine, when pro-Palestinian students interrupted him with epithets and slogans. He was unable to utter more than a few portions of his remarks at a stretch, although he did ultimately finish the speech.

But that incident was widely condemned. Ten protesters were later found guilty of disrupting a speech and ordered to perform community service. Upholders of free-speech rights insisted that at an institution of higher learning, you don’t shout people down; a liberal education requires that all views be given a hearing. And when Oren’s critics countered that Israel’s policies in Gaza and the West Bank placed its defenders beyond the protections of academic discourse, they found themselves in an impossible position. Free-speech principles, after all, are either universal or they become politicized and diminished, subject to the whim of those in power.

In recent years, however, anti-normalization has provided a new justification for singling out Israel’s supporters for silencing. For decades, Israel’s detractors struggled in vain to rebut the point that they were unfairly targeting a relatively liberal democracy while ignoring the far worse human rights violations of numerous state and nonstate actors. Anti-normalization offered a convenient principle specific to the Israeli-Palestinian conflict — one that could create a rhetorical escape hatch from questions of why, by this logic, defenders of Iran, Saudi Arabia, Russia or China didn’t also deserve to be silenced.

As anti-normalization spread as a tactic, it acquired a higher status. Advocates of BDS — the campaign to boycott, divest from and sanction Israel — began to grant this “principle” a quasi-theological character, lending its application to campus events an air of moral urgency and ethical superiority. By last year, BDS supporters had a transcendent reason to voice their contempt for academic freedom when they refused to participate in “normalizing” dialogue about the Israeli-Palestinian conflict and to block campus access to speakers deemed sympathetic to Israel.

As a result, such incidents proliferated. In October 2015, former Israeli Supreme Court chief justice Aharon Barak, noted for his support of Palestinian rights, had his own UC-Irvine talk interrupted and curtailed. The following month the world-renowned Israeli philosopher and New York University faculty member Moshe Halbertal had a University of Minnesota lecture disrupted. In February, Israeli Arab Bassem Eid was relentlessly heckled by BDS activists at the University of Chicago; in April, they blocked Jerusalem Mayor Nir Barkat from speaking at San Francisco State University.

In other cases, anti-normalization prompted people to prevent a speech simply because it was co-sponsored by a Jewish student group. At Brown University in March, the transgender activist Janet Mock canceled a speech after 160 anti-Israel students objected because the campus Hillel chapter was among the sponsors.

Anti-Israel speakers have also faced calls to have their invitations rescinded. In 2013, the University of Michigan withdrew an invitation to Pulitzer Prize-winning novelist Alice Walker, who has compared Israel to Nazi Germany. In 2011, the City University of New York withdrew an honorary degree to playwright Tony Kushner, a fierce critic of Israel, only to quickly reinstate it. These incidents, too, are completely unacceptable, but — significantly — they were one-offs, not the result of a policy espoused by an international campaign.

The growing practice of silencing pro-Israel speakers — of denying them the right to be treated as equals in campus debates — constitutes a dire threat to academic freedom. In our deeply polarized times, it is more important than ever that universities create opportunities for students and faculty to hear and engage with ideas that they don’t share. Their leaders must defend more vocally than they have thus far the free-speech rights of all speakers on the Israeli-Palestinian conflict. They must do so now, before the shouting down of unpopular views becomes, for lack of a better word, normalized.