Can grassroots activists stop Trump? Watch Phoenix this weekend.

In recent weeks, we have seen an appalling rise in violent attacks on protesters and journalists at Donald Trump rallies. The Republican frontrunner has not merely tolerated this political violence, but actively encouraged it. Remarkably, given the very real physical threats activists encounter, anti-Trump protests have over time become larger and more organized. The weeks to come will reveal whether this popular resistance to Trump’s xenophobic rhetoric can do more than shut down a rally. But confrontational young and minority activists have already redirected the substance of the 2016 election once. There is good reason to imagine they can do so again.

Protesters have been a part of Trump events from the beginning, and violence against protesters dates at least to the fall. By early March, protesters were staggering their disruptions in “an almost constant stream.” In Chicago, protesters numbered in the hundreds, organized in part by University of Illinois-Chicago student groups and the progressive organization MoveOn—and Donald Trump opted not to show.

Donald Trump claimed he had to cancel the event in response to the threat of violence, and then bemoaned the violation of his First Amendment rights. This is a truly Orwellian piece of logic. Donald Trump explicitly calls for violence against protesters and then claims his rights were being violated by the threat of violence that he has encouraged. As is only typical, he finessed this incoherence with a racially-loaded slur, describing the college students as “thugs.”

But there is even a deeper dishonesty here. Neither the Secret Service nor the Chicago police thought the event in Chicago posed a security threat. Those on the scene and responsible for the participants’ safety were taken aback by the event’s cancellation. Mr. Trump did not, despite his claims, cancel his event on the advice of law enforcement. He cancelled an event where his critics were too numerous to evict.

Let’s be clear: Mr. Trump’s free speech is in no way violated by protesters loudly objecting to his message. Free speech does not mean freedom from criticism. When the Klu Klux Klan marches on public streets, their right to do so is protected by the First Amendment. It is the right of others to drown out their message of hate. The Founding Fathers did not enshrine the right to a warm reception for one’s bad ideas. Trump’s rights are perfectly intact—what he is missing, apparently, is the courage to face an unfriendly audience. (To give you a sense of his concern with the First Amendment, Trump’s commitment to excluding anyone who might challenge him extends to a growing number of journalists.)

As usual, however, loud protests by brash young activists have provoked a spate of media tsking, along with the usual claims that their efforts were inadequately polite and therefore counterproductive. But as history has repeatedly shown, successful protest actions are often impolite. Protests are “divisive” in the best sense of the term—they demand observers abandon their apathy and take a position.

E. E. Schattschneider described this phenomenon as the socialization of conflict. In any conflict, the capacity to draw in observers on one’s side changes the power dynamic. Protest is a powerful way to involve and implicate the public in one’s concerns. A picket line is a useful example. When a union pickets a store, the customer can no longer avoid making a decision. Either they support the strikers, and shop elsewhere, or support the management, and cross the picket line. There is no longer the option of pretending the situation is “not your problem.” Protests remind us that politics is a game that does not have sidelines.

One clear effect of Chicago is the increasingly professional mobilization of anti-Trump activism. As of early Tuesday, leaders of the liberal establishment, including SEIU, NARAL, and the Sierra Club, are calling for a “massive nonviolent mobilization.” Trump’s planned rally this weekend in Arizona will be a crucial proving ground.

If we are seeing the start of a coherent and effective opposition to Donald Trump, we should remember where it started. Politicians across the political spectrum have described Donald Trump as dangerous. But in the face of Donald Trump’s explicit racism and xenophobia, and even his calls for violence against his critics, elites have been hedging their bets and passing the popcorn. It is (largely young and minority) grassroots activists who for weeks and months have risked their safety to take a stand against bigotry.

Now we get to choose whose side we are on.