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If Donald Trump keeps being a bully on Twitter, someone’s going to get hurt.

Last night, Chuck Jones, president of United Steelworkers 1999, was quoted in a CNN segment saying that the president-elect had “lied his ass off” about the deal to save jobs at the Carrier plant in Indiana. According to Jones, only 730 union jobs were being saved (Trump had given the figure of 1,100, but that included non-union jobs that were not going to be out-sourced).

Trump responded with two petulant tweets attacking Jones and the union:

Trump has more than 17 million Twitter followers, and some of them took Trump’s words as a call to harass Jones in real life. As The Washington Post reports:

Half an hour after Trump tweeted about Jones on Wednesday, the union leader’s phone began to ring and kept ringing, he said. One voice asked: What kind of car do you drive? Another said: We’re coming for you.

He wasn’t sure how these people found his number.

“Nothing that says they’re gonna kill me, but, you know, you better keep your eye on your kids,” Jones said later on MSNBC. “We know what car you drive. Things along those lines.”

The incoming president of the United States is using social media to launch personal attacks against private citizens that have criticized him. This behavior, one of the many ways that Trump is violating the norms that govern his office, could lead to real violence.