It took me a long time to understand why Donald Trump’s nasty tweets about people he didn’t like mattered at all. Protecting free speech means that people, including candidates for office, are allowed to say stupid hateful things. I didn’t realize that Trump’s persistent threats to unleash his “beautiful” Twitter account was something that could be feared.

But, it doesn’t take many of Trump’s millions of Twitter followers to unleash a hell on the momentary targets of his wrath. Megyn Kelly, of Fox News, now reports that her questions about Trump’s attitudes and conduct toward women won her a series of juvenile insults from the bloated bully. But it wasn’t just name-calling: Trump supporters alerted to their candidate’s irritation with her joined in to torment the television personality. Some of the harassment involved nastier comments and threats of violence on Twitter, but it didn’t end there.  Kelly says that she was stalked, and strange men occasionally turned up at her home; her children were terrified. In a recent interview promoting her book, she says:

It was my year of guards and guns — you know, thanks to Trump. I was under security threat for most of the nine months he was really coming after me. I had strange people showing up at my house. I had strange people casing my house. I had my children looking out the windows afraid. … Every time he would come after me, he would release — as I describe in the book — a torrent of nastiness in my life, and I had to sort of just be steady at the helm, because I was going to cover this race come hell or high water.

Kelly says that colleagues at Fox News tried to intervene with Trump, but the mogul demanded an apology from her. Although Kelly adamantly denies apologizing, she explains that she trekked to Trump Tower to discuss the issue; satisfied by this act of contrition, Trump stopped tweeting about her–for now.

Of course, this is awful, but we might think that people working in the public eye, like journalists, union leaders, or candidates for office, knowingly take on some risk.

That should not be the case for citizens who engage in political campaigns. In October 2015, when only the extraordinarily prescient or gullible thought Trump might be elected president, Lauren Batchelder, then an 18 year-old college student, asked him tough questions about women at a campaign event in New Hampshire. After the event, Trump tweeted an attack, and his followers quickly posted her name and address. For the past year, Batchelder’s social media accounts filled not only with criticism, but with explicit threats of violence. Again, it doesn’t take many crazy people to make someone’s life awful, and maybe to warn others off asking hard questions, or even going public in opposition.

Trump’s threats to unleash Twitter on his enemies suggest that he has some idea of what a sociopathic sliver of his followers are willing to do–and that he’s ready to intimidate would-be critics.

The violence, real and threatened, is an important dimension of fascist movements. I’d thought that the comparisons between the Trump campaign and noted fascists of the past, especially Hitler, were overblown and alarmist. I still do—but:

In the earliest days of the Nazi Party in the early 1920s, brown-shirted storm-troopers (SA) guarded party rallies–and violently disrupted the public meetings of rival parties, and fought in the streets with political opponents. Recruiting unemployed men, often veterans of World War I, the Party maintained control over its paramilitary forces, dispensing military titles and establishing a disciplined hierarchy. The SA gave frustrated and hungry young men discipline, a sense of belonging, and something to do that helped the Nazi Party. After coming to power, however, Hitler sought to consolidate the control of street violence within the state, obviate populist demands for somewhat socialist policies, and gain the support of the army and industrialists. In 1934, on the Night of the Long Knives, he had the leadership of the SA, including long time allies, arrested and executed.

The thugs motivated by hostile tweets differ from the SA in so many obvious ways: organization, coordination, and size (at its height, the SA numbered more than 3 million men) just to start, but the threat of violence can intimidate opposition and still political debate. This can’t be acceptable.

To date, there’s no hint that Trump, now elected president, has any intention of tempering his twitter threats, nor even picking his targets more selectively. But it’s hardly crazy to worry that a wayward tweet might lead to a violent outcome. Knowing the risks means taking responsibility.