President-elect Donald Trump’s Twitter account isn’t just a way for him to sound off. It’s a weapon he can use to exact revenge against his critics, including ordinary people, by sending mobs of his fans after them.

A tweet from Trump is enough to ensure that his target’s phone will start to ring with vague threats from strangers. His or her inbox will fill up with explicit messages and invective. If the harassment gins up media coverage, that ensures more of it. For one target, the messages from Trump fans have lasted at least a year.

Celebrities can’t always control their online followings. But Trump isn’t just a celebrity anymore. He’s the president-elect. And by now, he should know that a single tweet is enough to send followers to silence his critics, whether they’re an 18-year-old college student or a local union leader.

Melania Trump said before the election that her cause as first lady would be to prevent cyberbullying. But cyberbullies have become Trump’s shock troops, and their harassment doesn’t just stay on Twitter.

So far, Trump has done little to stop them — perhaps because he knows that to create a chilling effect for would-be critics, he doesn’t need to wield the formal powers of the government. He just needs to send a tweet.

A college student criticized Trump. His supporters attacked her for a year.

Lauren Batchelder, then an 18-year-old college student, had no idea in October 2015 that challenging Donald Trump would lead to more than a year of frightening harassment. But after Trump tweeted about her the next day, that’s exactly what happened, the Washington Post’s Jenna Johnson reported Friday.

At the bipartisan “No Labels” conference in October 2015, Batchelder said: “So, maybe I'm wrong, maybe you can prove me wrong. But I don't think you're a friend to women.” After Trump began to respond, she continued: “If you become president, will a woman make the same as a man, and do I get to choose what I do with my body?”

Batchelder had volunteered for the Jeb Bush campaign, but she wasn’t a paid staffer. The Bush campaign told the Washington Post it hadn’t sent her to Trump’s event. Nonetheless, Trump attacked her as a “plant,” saying she was “arrogant” and had questioned him in “a nasty fashion”:

The arrogant young woman who questioned me in such a nasty fashion at No Labels yesterday was a Jeb staffer! HOW CAN HE BEAT RUSSIA & CHINA? — Donald J. Trump (@realDonaldTrump) October 13, 2015

Trump didn’t even mention Batchelder by name, but she was named in media coverage of the event, and that plus Trump’s tweet was enough: The day after Trump tweeted, Batchelder told the Washington Post, she began getting threatening, sexually explicit calls, voicemails, emails, and Facebook messages. Her address was published online.

And long after the rest of the world forgot that there had even been a No Labels conference in Manchester, New Hampshire, let alone what anyone said, Batchelder was still getting threats, including this one, according to the Post: “Wishing I could fucking punch you in the face. id then proceed to stomp your head on the curb and urinate in your bloodied mouth and i know where you live, so watch your fucking back punk.”

Trump knows the effect his Twitter attacks have — and he’s still tweeting

Twitter gives Trump a veneer of plausible deniability: He’s not asking anyone to attack his critics, he’s just airing his grievances. But Trump should know by now what happens next — because his supporters’ online harassment was cited in a lawsuit against him.

In January 2016, a few months after Trump’s encounter with Batchelder, Republican consultant Cheri Jacobus criticized Trump’s decision to skip the final debate before the Iowa caucuses. Trump, who had already tweeted angrily about Jacobus in December, responded on Twitter:

Great job on @donlemon tonight @kayleighmcenany @cherijacobus begged us for a job. We said no and she went hostile. A real dummy! @CNN — Donald J. Trump (@realDonaldTrump) February 3, 2016

Then he renewed the attack a few days later:

Really dumb @CheriJacobus. Begged my people for a job. Turned her down twice and she went hostile. Major loser, zero credibility! — Donald J. Trump (@realDonaldTrump) February 6, 2016

Jacobus wasn’t doing anything out of the ordinary when she criticized Trump on CNN. Analyzing candidates’ strengths and weaknesses and commenting on their campaigns is why consultants go on TV. Most politicians understand that, and don’t waste time picking fights about these criticisms. But Trump did.

For more than a month, as the New York Times reported, Trump’s supporters responded with online abuse — sexist attacks that accused her of being an obsessive, Fatal Attraction–esque spurned job seeker, including a meme that depicted Trump gassing Jacobus in a gas chamber. Trump supporters bought up CheriJacobus.com and other domain names, creating websites attacking her as unprofessional.

Jacobus sued Trump for libel, saying she’d never sought a job with him. (Instead, his campaign had reached out to her, and she’d declined, she said.) Her lawsuit included details, including screenshots, about the online harassment.

Regardless of whether Jacobus is likely to win the lawsuit, the fact that Trump’s tweets called down harassment on her seems incontrovertible — but the president-elect is still tweeting. On Wednesday night, he personally attacked Chuck Jones, a steelworkers union local president in Indianapolis, who represents the employees of the Carrier factory in Indiana that was persuaded not to move to Mexico.

Trump claimed he saved 1,100 jobs at the Carrier factory. The actual number was only 800, and Jones told the Washington Post he thought Trump had lied to Carrier employees. In response, Trump tweeted:

Chuck Jones, who is President of United Steelworkers 1999, has done a terrible job representing workers. No wonder companies flee country! — Donald J. Trump (@realDonaldTrump) December 8, 2016

Jones started receiving threatening messages, including some in which the callers said they knew he had children.

Trump’s tweets aren’t just scary — they’re interfering with First Amendment rights

Trump seems to think that any time he’s insulted, no matter who does it, he’s entitled to respond in kind. And given that he won the presidential election even though lashing out at private citizens was considered a threat to his campaign, he may be feeling even more empowered than he did in the past to take on his critics — no matter who they are.

This isn’t just a startling break with precedent. It’s a real threat to free speech.

A Pew Research Center study in 2014 found that of people who had experienced severe harassment — including physical threats or harassment for an extended period of time — online, 21 percent either changed their usernames or deleted their profiles entirely, and 12 percent altered their offline behavior in response to online harassment.

In November, a study from the Data and Society Research Institute found that 27 percent of all Americans had censored themselves, deciding not to post something on the internet because they feared online harassment in response.

In other words, online harassment is a potent tool to get people to alter their behavior.

There’s a term for what happens when people don’t exercise their legal rights — like criticizing the president — because they fear the consequences. It’s called the “chilling effect,” and the Supreme Court considers it as a real concern when making decisions related to the First Amendment.

The specter of a president using his power to silence critics usually conceives of power in formal, governmental terms — a president sending the FBI to investigate his political enemies, nudging the IRS to audit them, or even (as has happened in the past in the US) arresting and imprisoning them.

Trump, though, has proved that idea is outdated. He doesn’t need any of that to make people think twice about criticizing him. All he has to do is tweet.