GOP presidential front-runner Donald Trump has a reputation for playing rough with his opponents, including the handful of protesters who now regularly show up at his rallies to voice their disgust at his policies. He has shown every indication of relishing these confrontations, often getting cheers from the crowd for his displays of contempt and ridicule. The situation in recent days has clearly gotten out of hand — protesters pepper sprayed by police in Kansas City, security surrounding the candidate as a man jumped on stage in Ohio, a rally canceled in Chicago out of fear that confrontations between protesters and supporters would turn violent. His opponents are now openly accusing Mr. Trump of inciting violence through his incendiary remarks, but by now it appears too late to change the campaign's trajectory.

Graphic evidence of that surfaced last week when a Trump supporter physically assaulted a heckler in the crowd, and the candidate seemed to condone the attack. By refusing to unambiguously condemn the assailant, Mr. Trump set a dangerous precedent that sooner or later could get someone seriously injured or killed.

The incident occurred during a campaign rally in Fayetteville, N.C., where a group of activists for the Black Lives Matter movement that included a white woman, a Muslim and a gay man loudly interrupted the candidate several times during his speech. Eventually, Mr. Trump asked security guards to escort the hecklers out of the hall after the crowed began booing their disruptive presence. What happened next was captured on video footage showing two uniformed officers leading a young black man in a T-shirt toward the door when suddenly an older white man in a cowboy hat rushed into the frame and sucker-punched the protester in the face, knocking him to the ground.

The alleged attacker, identified in news reports as John McGraw, later expressed satisfaction over what he had done to activist Rakeem Jones, telling an interviewer that the best thing about it was “knocking the hell out of that big mouth.” Mr. McGraw went on to add that “we don't know who he is, but we know he's not acting like an American. The next time we see him, we might have to kill him.” (The next day the Cumberland County sheriff's office announced Mr. McGraw had been taken into custody and charged with assault and disorderly conduct in connection with the incident.)

Yet Mr. Trump, who was still standing on the podium as the scene unfolded, seemed unperturbed by the violence. His response was of a piece with the candidate's earlier apparent indifference to such incidents, and indeed of his own oft-expressed desire to physically confront his detractors. At a campaign event in Nevada last month Mr. Trump said of a protester there that “I'd like to punch him in the face,” and last year he dismissed a heckler involved in a scuffle with police in Birmingham, Ala., by saying, “Maybe he should have been roughed up.” In Dayton, Ohio, over the weekend, when a man jumped the stage, Mr. Trump bragged that he “was ready” but that it was probably better to let the police handle it. Mr. Trump has also offered to pay the legal expenses of people arrested for assaulting hecklers at his events in Kentucky, where former Towson University student Matthew Heimbach, who founded a “White Student Union” at the school, was involved in a shoving match earlier this month with protesters in Louisville. In Kansas City this weekend, he urged police to arrest protesters whether they were violent or not.

Mr. Trump is known for his bombastic rhetoric and take-no-prisoners style of attacking those who disagree with him, and that tactic is reaching its inevitable conclusion. He's not just whipping up partisan fervor but actually directing his audiences' emotions in a way that amounts to an incitement to violence. There's a dangerous undercurrent of bigotry just below the surface of Mr. Trump's speeches that speaks to the worst instincts of his supporters, and once aroused it's likely that even he can't control the raw and angry passions his words unleash.

Mr. Trump may say he simply is refusing to be muzzled by “political correctness,” but what he is really doing is flirting with a kind of anarchy.

Yet if, as appears possible, Mr. Trump goes on to win the GOP presidential nomination, the general election campaign may only amplify this disturbing tendency. There will be many opportunities for protesters to interrupt GOP campaign events between the party convention this summer and the November election, raising the real possibility of violence if Mr. Trump continues in his current vein. If the Trump candidacy devolves into a campaign where fistfights and attacks on hecklers become the norm, Mr. Trump, his party and the country will all suffer.