Friday was an ugly day on the campaign trail, perhaps the worst of the year. What erupted in St. Louis and fully boiled over later in Chicago, however, was no aberration. Donald Trump has built his candidacy on long-festering resentment and grievance. It is a poisonous combination, for the Republican Party and for the country.

Trump’s slogan is Make America Great Again, but his campaign for president continues to call out dark forces that divide a polarized America. Fueled by acrimonious rhetoric, he has sparked an angry movement that has now created an angry backlash. Campaign 2016 is on a downward and dangerous descent.

The videos of the conflicts ahead of Trump’s rally in Chicago on Friday evening triggered memories of the far-worse bloody clashes at the 1968 Democratic convention in the same city, when the United States was engulfed by violence and protests over the war in Vietnam. The flash points in today’s politics are more diffuse, but the political divisions are no less real.

Trump seems unwilling to try to put the genie back in the bottle. Even if he were, it’s questionable that he could. Anger is the fuel that feeds his candidacy. Passions on both sides intensify by the week. Scattered protests at his rallies have escalated into terrifying confrontations. Violence is more commonplace.

Standing in the airplane hangar at Trump’s rally here in Ohio on Saturday morning was Tom McMurtry, 60, a community college police officer and Iraq War veteran. He described himself as a former Republican who is now an undecided independent who wanted to hear what Trump had to say. He watched Friday’s melee in Chicago as he was eating dinner.

Amid growing security concerns, Republican presidential candidate Donald Trump's campaign canceled a Chicago rally on March 11. (Victoria Walker/The Washington Post)

Did he blame Trump for any of this? “He could certainly have done things to calm things down, but a lot of his appeal is that he gets people riled up. He stirs people up,” McMurtry said. “It’s hard to stir people up and then at the last possible instant tell them to stop. It’s a momentum heading toward violence, and last night it hit people moving in the other direction.”

[The melee in Chicago at Trump’s rally]

In the aftermath of his canceled rally in Chicago, Trump was unrepentant, even defiant, about his role. He was asked during phone interviews on cable TV late Friday, as clips of the violence played on the screen, whether he regretted the kind of pugnacious rhetoric he has used that might have encouraged violence against protesters. He declined to offer any remorse.

On Saturday morning, en route to his rally here in Ohio, he tweeted, “The organized group of people, many of them thugs, who shut down our First Amendment rights in Chicago, have totally energized America!”

An hour later, as he opened his rally, where the crowd was so large that it filled the hangar and spilled onto the tarmac, Trump again defended himself and his followers. In Chicago, he said, his people came under an organized attack. “We cannot let our First Amendment rights be taken away from us,” he said. “We want to get along with everybody.”

At one point during Saturday’s rally, someone jumped a barrier behind Trump and tried to get on the stage. Secret Service agents rushed to surround Trump and the threat was quickly subdued. “I was ready for him,” Trump said to the cheering crowd, “but it’s much easier if the cops do it.”

There were other scattered protests, with the demonstrators led out of the hangar by law enforcement officers. “Get him out of here,” the candidate thundered into his microphone as one protester was escorted out. “Go home to mommy.”

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With each disruption, the atmosphere grew more intense, the audience cheering louder and louder, nearly drowning out the candidate’s voice. It was mild by Friday’s standards but indicative of what now envelops the Trump for president movement. The rallies are supercharged with energy and all that comes with it.

There is no condoning the actions of those on either side of the violence that erupted Friday or at other Trump events. No one is. But the Republican front-runner is in a minority position on the question of what has brought the presidential campaign to this moment. After all, it is Trump who, as protesters were led out of earlier rallies, stoked the anger of the crowd with crude comments about wishing he could punch out those who have disrupted his events.

[Racial tensions explode at Trump rallies]

Friday’s events produced a change in tone among Trump’s Republican rivals, who are desperate to stop his march to the GOP nomination. Earlier this month, they stood on a stage and said they would support him if he becomes the party’s nominee. After what took place Friday, they were quick to cast Trump as the enabler and facilitator of a climate of hate that now seems to surround his candidacy.

Ohio Gov. John Kasich has long resisted direct engagement with Trump. Late Friday he issued a blunt statement, saying, “Tonight the seeds of division that Donald Trump has been sowing this whole campaign finally bore fruit, and it was ugly.” On Saturday morning, he talked of the “toxic environment” that has been created by Trump’s candidacy.

Sen. Ted Cruz of Texas did not excuse the protesters who disrupted Trump’s rallies but held little back in blaming Trump for what is happening. Trump, he said, “disrespects voters,” “affirmatively encourages violence” and has a campaign “facing allegations of physical violence against members of the press.” In sum, he said, Trump has created an environment “that only encourages this sort of nasty violence.”

As of yet, neither Cruz nor Kasich nor Sen. Marco Rubio of Florida is willing to renounce his pledge to support the eventual nominee even if it is Trump, although Rubio said it gets harder by the day not to back away. That is their and their party’s dilemma.

[Trump rally in Chicago canceled ]

Trump is in a strong position to win that nomination and could be in an even stronger position after Tuesday’s primaries. Only a Kasich win in Ohio and a Rubio victory in Florida on Tuesday would make his path significantly more difficult. Under most plausible scenarios, he will have more delegates than any of the other three remaining candidates by the time the primaries end in June.

Republican Party leaders face the prospect of a convention in Cleveland riven by intraparty division over Trump’s candidacy and convulsed by what could be rounds of protests outside the arena. Things may seem toxic today. Imagine what they could be by then if he is on track to claim the nomination.

Some Republicans believe there can be eventual accommodation to a Trump nomination and that he would mellow once he has won the battle. Those who believe that no doubt were heartened by Thursday’s debate in Miami, when he showed a more restrained demeanor and avoided insulting his opponents. But his candidacy already has so fractured the party and the country that the climate isn’t likely to change.

Trump’s candidacy now is far more than a Republican Party issue. He defines and dominates the politics of 2016. He did not create the root causes of the anger and division in the country. All of that has been building for years, for all to see.

The problems are real and difficult to address. There has been little effort to contain or restrain the disaffection, particularly in the Republican Party. More than anyone, however, Trump has exploited that anger.

No one should underestimate the significance of the estrangement or disregard the implications of it. It was on full display Friday. Even worse could occur in the days and months ahead unless there is a collective and concerted effort to step back from this precipice.