Mr. Trump’s campaign declined to discuss its security precautions. It was clear, though, that the Trump sentries caught some but not all the protesters who passed before them, and that they also ensnared a reporter, who had no plans to disrupt anything, in their dragnet.

Ms. Roades was with 10 anti-Trump protesters who had planned to sneak in with signs that said things like, “No hate in our state” and “Hate is not a Wisconsin value.” But four of them were stopped at the door, Ms. Roades said, including herself.

Another one of those who were thwarted, Nathan Royko Maurer, 43, said he briefly made it inside but was ousted after taking a seat. Security approached him, he recalled, and said that he had been identified as a protester. “You’re going to have to leave,” they had said.

As it turned out, there was more action outside the site than inside. The Janesville police were looking for a man, who appeared to be wearing a red “Make America Great Again” baseball hat, who pepper-sprayed a 15-year-old protester who swung at a Trump supporter after, she said, she was groped.

There is a local flair to the Trump protest movement, with demonstrations largely carried out by activists on the ground at his various stops. But there is also some national coordination involving activist groups like Black Lives Matter, the Grassroots Global Justice Alliance, Mijente and Showing Up for Racial Justice, or SURJ.

Just this month, a group of activists formed the Stop Trump National Network to marshal those nationwide eager to block Mr. Trump’s White House bid.