Cheri Jacobus, a Republican political strategist, did not think she had done anything out of the ordinary: On a cable television show, she criticized Donald Trump for skipping a debate in Iowa in late January and described him as a "bad debater."
But then Trump took to Twitter, repeatedly branding Jacobus as a disappointed job seeker who had begged to work for his campaign and had been rejected. "We said no and she went hostile," he wrote. "A real dummy!" Trump's campaign manager told the same story on MSNBC's "Morning Joe."
Trump's Twitter followers, who number about 6 million, piled on. For days, they replied to his posts with demeaning, often sexually charged insults aimed at Jacobus, including several with altered, vulgar photographs of her face.
"Cheri is a nutcase," wrote @LegendaryTrump. Another Twitter user, @stockedwood, wrote, "How the hell does a woman like her get on the air???"
With his enormous online platform, Trump has badgered and humiliated those who have dared cross him during the presidential race. He has latched on to their vulnerabilities, mocking their physical characteristics, personality quirks and, sometimes, their professional setbacks. He has made statements, like his claims about Jacobus, that have later been exposed as false or deceptive — only after they have ricocheted across the Internet.
Many recipients of Trump's hectoring are fellow politicians, with paid staff members to help them defend themselves. But for others, the experience of being targeted by Trump is nightmarish and a form of public degradation that they believe is intended to scare off adversaries by making an example of them.
And as more mainstream forces in the Republican Party try to unite to stop his march to the nomination, they are finding that the fear of taking on Trump is discouraging his would-be opponents from signing on to the fight.
"I've never encountered an American politician at this level that people are literally afraid of — donors are afraid of him," said Rich Lowry, editor of National Review, a magazine that has criticized Trump. He added, "If it was Hillary Clinton that was doing it, the entire right-wing world would erupt in outrage, understandably and correctly."
It is not just that Trump has a skill for zeroing in on an individual's soft spot and hammering at it. It is that he sets a tone of aggression against the person, and his supporters echo and amplify it.
"Over and over and over again: You were rejected by him, you're a scorned woman," Jacobus said, paraphrasing the hundreds of online attacks she faced.
Jacobus sent a cease-and-desist letter to Trump and his top aide, citing electronic messages that showed the Trump campaign had courted her and not the other way around.
"I have been trashed and ruined on Twitter," Jacobus said. She said Trump's lawyers had responded to her letter but had not reached a resolution.
Even when Trump moves on to something or someone else on Twitter, his followers linger on the last fight.
Lowry, who devoted an issue of his magazine to critiquing Trump and has welcomed the candidate's scorn, said he received a flood of hostile Twitter attacks from Trump's followers after the issue was published. Their posts, he said, regularly include "some really vile, neo-Nazi-issue white nationalist, heinous personal abuse, kind of racially tinged stuff."
As Trump has risen in the Republican race, he has only dialed up his tactics. This week, he sent out a menacing message on Twitter
about the Ricketts family, a wealthy clan of Republican political donors, after it was reported that Marlene Ricketts donated $3 million to a group opposed to Trump's candidacy.
"They better be careful," Trump wrote of the family, "they have a lot to hide!"
"It's a little surreal when Donald Trump threatens your mom," Marlene Ricketts' son, Tom, later told reporters.
Others say Trump's actions go beyond the outlandish and cross into more sinister territory. Parry Aftab, a lawyer who leads the Internet safety group WiredSafety, said Trump's behavior was a textbook example of cyberbullying.
In particular, she said his methods were characteristic of "mean-girl cyberbullying" because he enlists others to mimic his attacks. She said his conduct resembled the violent and abusive language her organization can often get removed from Facebook and Twitter.
"At what point does it cross the line into something that's defamatory and might be actionable?" Aftab said. "At what point does it cross the line into encouraging violence against groups and individuals?"
In Iowa and New Hampshire, Trump savaged local power brokers who had endorsed other candidates and criticized his campaign. He called Joseph W. McQuaid, publisher of The New Hampshire Union Leader, a "psycho" and a "dirty dog," and he accused him of trying to force Trump to advertise in his newspaper before it endorsed Gov. Chris Christie of New Jersey.
McQuaid, who has roundly denied Trump's claims, said he still gets occasional Twitter messages and letters from readers about his clash with Trump, weeks after New Hampshire's Feb. 9 primary.
"I think his goal," McQuaid said, "is to shut down opposition."
Corey Lewandowski, the Trump campaign manager, said his candidate's practice of battering opponents on social media showed that Trump was "the ultimate counterpuncher," a tough candidate unwilling to take even the slightest criticism lightly.
"When someone attacks him, should he just not respond?" Lewandowski said. "That's not fair."
Lewandowski said he was unaware of a cease-and-desist letter from Jacobus, and he described the Trump fans who follow the candidate's lead as people who "trust Donald Trump to, when he has to, take action" against those who wish him ill.
If Trump's merciless methods have seemed to help him so far, there could be a political downside over the long run. His attacks have opened wounds in the Republican ranks that are unlikely to heal anytime soon. Several people he has targeted have said they would have difficulty supporting him in a general election.
Bob Vander Plaats, an influential evangelical activist in Iowa, said it might be impossible for him to vote for Trump. Trump wooed Vander Plaats for his endorsement early on and turned on him bitterly after Vander Plaats instead backed Sen. Ted Cruz of Texas.
In late January, Vander Plaats recalled, he was about to go on television when his phone beeped, as it does whenever his Twitter handle is mentioned. This time, the mention was by @realDonaldTrump, who accused Vander Plaats of seeking free hotel stays for his family in exchange for his endorsement of Trump. Vander Plaats said the charge was flatly false.
"I mean, the wind gets knocked out of you," Vander Plaats said. "And the reason the wind gets knocked out of you is you think: This is going out to millions of people. And your mom's going to read it. And it's going to be in the newspapers."
Asked if he could ever vote for Trump, under any circumstances, Vander Plaats paused before answering, "It would be very difficult, today, to say I could cross that bridge to get there."