IT MUST be painful for President Barack Obama to see some of the worst incidents of racial tensions and violence of his presidency erupt during its last few months. On March 12th, in offering his assessment of the violent clashes between supporters of Donald Trump and protesters, the president struck a sombre tone. Those who aspire to be our leaders should be trying to bring us together, speak out against violence and reject efforts to spread fear, he said at a fundraiser in Dallas. “And if they refuse to do that, they don’t deserve our support.” Mr Obama didn’t mention Mr Trump by name. He didn’t have to. The day before his speech in Dallas, the increasingly ugly violence at Trump rallies induced a paroxysm in Chicago, the president’s adopted hometown. Mr Trump was scheduled to speak at the nearly 10,000-seat pavilion of University of Illinois at Chicago (UIC), which has a racially diverse student body and is located on the ethnically mixed west side of a city that is a Democratic stronghold. He never showed up in the lion’s den, deeply disappointing thousands of his red-capped supporters, who had queued for hours to hear him speak. His no-show was greeted with jubilant victory chants and applause by the hundreds of protesters who had infiltrated the event, many of them UIC students, some with Bernie Sanders signs, others wearing T-shirts emblazoned with “Black Lives Matter”. Yet their successful shutdown of the event may turn out to be a Pyrrhic victory.

Accounts as to why the rally was cancelled diverge. Undisputed is that the waiting crowd grew increasingly confrontational as the factions of protesters and Trumpistas faced off against each other. Occasional scuffles and fistfights broke out. In one of the many striking photographs of the evening, an elderly woman wearing a Trump T-shirt lifts her arm in the Nazi salute. Interviewed with her husband afterwards, Birgitt Peterson, who was born in Berlin in 1946, explained that she was teaching protesters, who had taunted her and her husband as Nazis and skinheads and imitated the salute, how to “Heil Hitler” properly.

After letting the crowd wait for more than half an hour, a Trump staffer came out and said that Mr Trump “has determined that for the safety of all of the tens of thousands of people that have gathered in and around the arena, tonight’s rally will be postponed to another date”. Almost immediately after cancelling the event, the TV-savvy Mr Trump got in touch with MSNBC, a news channel, to give his version of events. “I felt it was just safer. I don’t want to see anybody get hurt,” he said live on air. He insisted that the Chicago Police Department (CPD) had advised him to call it off. The CPD said later it had done no such thing nor had it been consulted by the Trump team.

Why did Mr Trump choose such hostile terrain for his rally, just a few days before yet another important series of primary elections on March 15th? In the days before, online petitions and messages on social media indicated that Mr Trump would not get a friendly welcome. Over 10,000 said on Facebook that they would protest against his event. More than 300 worried UIC staff members signed a letter imploring the head of the university to cancel the event. The letter mentions recent instances where protesters have been pushed and punched before being escorted from Trump rallies. On the day before the planned rally in Chicago, a 78-year-old man in North Carolina was charged with assault for punching a black protester as he was removed from a Trump rally. “He deserved it,” said the unrepentant assailant in a televised interview after the confrontation. “Next time, we might have to kill him.” Mr Trump said he is toying with the idea of paying the legal fees of the septuagenarian.

Mr Trump may have taken the calculated risk of disappointing thousands of his supporters in the Chicago area in the safe knowledge that his rally would get far more attention by not happening. Some compared the scenes of Friday night to the violent clashes in Chicago during the Democratic National Convention in 1968, when protests were directed at the political establishment responsible for the war in Vietnam. This comparison suits Mr Trump: the bedlam of 1968 swept into power Richard Nixon, a hardline Republican whose campaign tactics and slogans bear some resemblance to Mr Trump’s.

The question now is whether the Chicago rally is a turning point that puts a definitive end to any of the dreams of a post-racial society that were inspired by America’s first black president. Some think the Trump movement is the last hurrah of Americans who regret the end of slavery. Other black residents of Chicago think this teleological view is too optimistic; they argue that bouts of racism are cyclical and that America will always live with the stain of its original sin.

Most agree that the aggression and provocation will not end and may yet grow uglier. Disturbances continued on March 12th at a Trump rally in Dayton, Ohio, where a man charged towards Mr Trump’s stage and was quickly surrounded by security personnel. “I was ready for him, but it’s much easier if the cops do it,” said the ever-combative Republican front-runner. On March 13th Mr Trump threatened to send his people to disrupt Sanders rallies. In an early-morning message Mr Trump tweeted “Bernie Sanders is lying when he says his disruptors aren’t told to go to my events. Be careful Bernie, or my supporters will go to yours!”

The other Republican contenders for the presidency have now, belatedly, decried the violence at Mr Trump’s rallies and distanced themselves from the candidate himself. John Kasich, the governor of Ohio, accused Mr Trump of creating a “toxic” environment in the race. Ted Cruz, a senator from Texas, said he was troubled by the rallies in which Mr Trump asks people to pledge allegiance to him by raising their hand. Marco Rubio of Florida, like Mr Kasich, seemed to waver on his promise to back Mr Trump if he were to become the Republican nominee. ”We are being ripped apart at the seams now, and it’s disturbing and I am sad,” said Mr Rubio. “This country is supposed to be an example to the world.”