You know, somebody's going to get killed if this keeps up.

What might be called the Maxine Waters effect arrived in Philadelphia on Monday morning, accompanied by whistles, bullhorns, and obscenities directed at a couple of civilians and a gaggle of police.

Maxine Waters is the volatile California congresswoman who has called for citizen vigilantes to get in the face — and ears — of members of the Trump administration when they are out in public. Several Trump officials already had been called out during meals.

This time, it was not Trump staffers, but Trump acolytes Charlie Kirk and Candace Owens, who were confronted as they ate breakfast at the Green Eggs Cafe on 13th Street near Locust. Antifascist protesters surrounded the restaurant after they learned Kirk and Owens, leaders of Turning Point USA, an organization for conservative college students, were eating there. It was "8:20 on the dot," according to restaurant manager Malik Joe.

How did the protesters find out they were there?

Progressive pundit and radio commentator Christopher Hahn, who had discussed this on Laura Ingraham's Fox News show, told me he suspected Kirk and Owens might have "staged" the protest.  Owens was later quoted as saying she and Kirk were recognized by some restaurant customers, antifa members, who then called their pals.

The protest outside the Green Eggs Cafe, according to published accounts and video, was noisy, with one protester dumping a glass of water on Kirk, which breaks the "no touch" rule. That was illegal.

While the protest was going on, Owens, who is the Turning Point communications director, described antifa as "an all-white fascist organization" (it is not) that "attacked" police (it did not), who were all African American or Hispanic (they were not).

In a video, the protesters can be heard chanting about "white supremacy" to Owens, who is African American. The protesters also indulged themselves in some stereotypical anticop rhetoric.

Philadelphia police monitor antifa groups' social media and know where they plan to protest. Kirk didn't respond to my request for an interview.

Aside from getting a little wet, which Kirk brushed off, there was no real damage, and Kirk declined to file charges against the person who doused him.

That was this time, but what about next time? I am looking ahead. Someone's going to feel threatened and pull a gun.

We have seen clashes between antifa and right-wing groups that have resulted in violence, such as in Portland and, of course, Charlottesville. Tempers are getting hotter, not cooler, as each side thinks it is saving America from the other side.

I don't accept antifa groups as part of the normal American left, which generally puts its faith in nonviolence. I don't agree that Trump's policies permit resorting to violence. When you show up at a protest with a hockey helmet on your head, a bandanna over your face, and a bat in your hands, then you are no better than that which you oppose.

I'm not trying to be alarmist, but it doesn't take much imagination to conjure up scenes from the 1960s of riots, street combat, and bombing. It did happen here and it can happen here again.

"Peaceful protests are generally a good thing," says Hahn, "but ambushing people in a restaurant is uncalled for and not consistent with our democratic society."

Let's keep someone from getting killed.