In foreboding conversations across the political world this past year, a bipartisan chorus warned that the 2016 presidential campaign was teetering on the edge of violence.
The anger from both sides was so raw, they concluded — from supporters of Donald Trump who are terrified they are losing their country and from protesters who fear he is leading the nation down a dark road of hate — that a dreaded moment was starting to look inevitable.
"I don't see where that anger goes," historian Heather Cox Richardson predicted a few weeks ago, "except into violence."
This weekend it finally arrived.
The ugly and chaotic clashes that unfolded Friday inside a steamy Chicago convention hall between Trump supporters and a coalition of protesters were the culmination of an extraordinarily indignant year in public life in which those on both sides of a widening divide have begun to see their fellow Americans as a fundamental threat to their economic future and basic dignity.
On Saturday night in Kansas City, police used pepper spray twice to disperse protesters outside Trump's rally.
By Saturday, it was clear that the past 48 hours were something of a turning point in the presidential race. Demonstrations at Trump rallies persisted, leading to a panicked moment in a suburb of Dayton, Ohio, when Secret Service agents briefly encircled the candidate after a man leapt over a security barrier and rushed toward the stage.
And Trump's rivals in both parties denounced his candidacy as the match that lit the fire, even as they try to harness the same electoral forces that have turned him into the Republican front-runner.
"Donald Trump has created a toxic environment," a visibly agitated Gov. John Kasich of Ohio declared. "There is no place for a national leader to prey on the fears of people."
Sen. Marco Rubio, fighting for his political life in Florida's primary Tuesday, likened Trump to a third-world strongman. Hillary Clinton accused Trump of committing "political arson," saying that "the ugly, divisive rhetoric we are hearing from Donald Trump and the encouragement of violence and aggression is wrong, and it's dangerous."
Inside a campus pavilion at the University of Illinois at Chicago on Friday, a tense night of pushing, shoving, sign-ripping and yelling left three people injured, authorities said, and at least four were arrested. Trump canceled the rally for safety reasons, and Saturday, sounding annoyed, he called the demonstrators "a disgrace if you want to know the truth," suggesting it was an organized protest with "professionally" made signs. (Activist groups did try to disrupt the event, but many protesters said that they learned of the demonstrations on social media and went of their own accord.)
Presidential campaigns have long flirted with the lexicon of violence, as candidates vow to take the country back from the opposing party in the White House and reclaim an endangered vision of America. But this year's campaign has distinguished itself by the sheer volume of heated words, led primarily by Trump, and by actual scenes of physical confrontation.
Now both Republican and Democratic leaders are predicting a long, grim and pugnacious phase of the presidential race.
"I've got to believe it's only going to get worse," said William M. Daley, the son of Chicago's famed mayor, Richard Daley, who presided over the violent 1968 Democratic convention. "Both sides are fueling this," he added.
Behind the showdowns is a climate of frustration and fright not seen since the 1960s, or even the 1850s when, in the words of Joanne Freeman, a Yale historian who has studied violence in American politics, "each side was convinced that the other side was about to destroy America — or what they believed to be the fundamental essence of America — and each side totally alienated the other side."
In Chicago on Friday, such a determination seemed very much in evidence.
Michael Joseph Garza, a 27-year-old employee of a Chicago logistics company who is part Mexican and part Italian, had read about the Trump rally on Facebook and, after discussing his candidacy with his wife, felt compelled to protest it to make a point about immigration and tolerance.
"Even if Trump just ruins this country for four years, I can't go to my children and say I did nothing to try to stop him," Garza said.
It is the kind of deep-seated mistrust and alarm over an unspeakably bleak future that is also expressed by supporters of Trump like Denise Rubino, 50, a bartender from Concord, North Carolina.
She worries that an America without Trump at its helm would be "a disaster" and despairs that his enemies within the Republican Party will try to seize the nomination from him, as many have pledged to try. Should that happen, Rubino said, his voters might not hold back.
"I think they're going to uprise," she said. "Because that's undermining the political process."
In a testament to how vitriolic the campaign has become, a wide range of figures have pleaded for a lowering of the political temperature and the heated messages, warning that it would produce physical altercations, or worse. Trump's own team seems highly attuned to the possibility.
On Saturday, in a rally at an airplane hangar in Dayton, not long after he had mocked a protester being escorted out — "Go back to mommy," he said — a man jumped a security barrier and rushed toward the stage. Trump ducked his head, grabbing his podium with both hands before backing away.
One of Trump's personal security guards, who has worked for him for years, was the first to jump on stage. Three other men who appeared to be Secret Service agents leapt on stage, and all formed a ring around Trump, while other security grabbed the man, tackled him and then escorted him away.
Christine Todd Whitman, the former Republican governor of New Jersey and a Cabinet member in the administration of George W. Bush, said she has long feared the fury that Trump's words could arouse in his supporters and detractors alike.
"You can't dial back the emotions he's excited in people easily," she said in an interview. "There will be consequences for that."
She recalled Trump's provocative remarks about Mexicans last year.
"If you were told that Mexicans are rapists or criminals and you make assumptions and you are walking down the street and see them in your community," she said, trailing off. "People are going to do things."
Reporting was contributed by Julie Bosman, Amy Chozick, Monica Davey, Matt Flegenheimer, Maggie Haberman and Jeremy W. Peters.