Awful though it was that thugs smashed their way into the U.S. Capitol last week, it might do the United States good in the end. Here’s how.

Inspiring violence, which I study through my organization, the Dangerous Speech Project, doesn’t happen in one speech or tweet. It’s a steady insidious process that Donald Trump carried out for years until it was interrupted by last week’s display of what such language can do. The riot laid bare the urgent duty of Trump’s influential allies: to clearly repudiate his incendiary lies.

Arnold Schwarzenegger did it this weekend, in a video denouncing Trump’s rhetoric by describing his own childhood in Austria right after World War II, “surrounded by broken men drinking away their guilt over their participation” in Nazism. “It all started with lies, and lies, and lies, and intolerance ...” Schwarzenegger said. “I know where such lies lead.”

A witness in a trial at the United Nations Tribunal for Rwanda, after the 1994 genocide, also knew. Explaining how a radio station had groomed its listeners to commit and condone unthinkable violence, the witness said, “In fact, what RTLM [Radio Télévision Libre des Mille Collines] did was almost to pour petrol, to spread petrol throughout the country little by little, so that one day it would be able to set fire to the whole country.”

Trump set his crowd on fire last week by saying, “You will never take back our country with weakness.” But that’s not what really brought about the attack on the Capitol. It was the steady flow of Trump’s love and lies over the past five years. It started with his 2016 campaign, when he told the audience at a rally to ”knock the crap out of” protestors and said he would “pay the legal fees.”

Every time Trump has made an inflammatory, hateful, and/or false remark since, journalists and Democrats called it out. But not Republicans, with the rarest of exceptions like Gabriel Sterling. This brave, infuriated Georgia elections official demanded on Dec. 1 that Trump “stop inspiring people to commit potential acts of violence.” As Sterling predicted, “Someone’s going to get hurt, someone’s going to get shot, someone’s going to get killed, and it’s not right.”

He was correct. But he was alone as a Republican publicly calling out the president, and the predictable result came.

Dangerous speech is potent precisely because it is gradual, so it’s easy to get inured to the dribbling, like a slow gas leak. The philosopher John Stuart Mill gave a famous but incomplete description of a single speech: a leader encouraging a hungry mob to break into the house of a corn-dealer and help themselves to the food inside. In D.C., the mob helped itself to Nancy Pelosi’s mail and other trophies. But they would never have done that if they had heard only one speech or even just a few.

Today we have a national oil spill, not genocide-inciting but violence-inciting. It is our urgent task as Americans to work on cleaning it up. Banning Trump from Twitter, Facebook, and other platforms will help to stop him from spilling more gasoline, or at least decrease the flow, but it won’t clean up what is already there in the minds of millions of his followers. And Trump’s opponents can try, but they can’t succeed. Trump’s allies can. Trump’s supporters need to hear his messages repudiated by other leaders they admire, such as ministers, media figures, and celebrities.

Almost all of those have so far remained silent, or have decried the violence, but not its catalyst. They should now explain that Trump was wrong to teach them to see Democrats, Muslims, immigrants, and journalists, among others, as their enemies.

Public censure is a stronger tool than we often imagine, and a vital one in this country where First Amendment jurisprudence (wisely) protects freedom of expression more than any other law in the world. It is strong enough to protect the republic — if the right people use it well.

Susan Benesch is the founding director of the Dangerous Speech Project and faculty associate at Harvard University’s Berkman Klein Center for Internet and Society.