In just the last week, Hillary Clinton has been rushed twice by people who had to be subdued, and Donald Trump has faced similar incidents in recent months -- not to mention a man this week climbing up Trump Tower using suction cups.
The Secret Service, which is primarily responsible for the security of the presidential candidates, takes precautions both seen and unseen to protect their safety. For example, at public events, they try to check all attendees for weapons using magnetometers or X-ray machines; they visually scan the crowd for abnormal behavior and potential bad actors; they set up a physical barrier to keep an 8-foot buffer space between the crowd and the stage; and they map out an escape path to a "hard room" backstage in case of trouble.
But if an individual from the crowd tries to rush the stage, former agents say their instructions are to stop an intruder by whatever means necessary.
"They have to assume that he's a threat, and they have to take him down," said former Secret Service Assistant Director Anthony Chapa. "He is attempting to approach the candidate, he's not been invited, he's doing it in a threatening manner, it's illegal to do so, and they are stopping him."
He says different agents have different responsibilities if someone rushes the stage: Those close to the candidate will shield the candidate and move the candidate to safety; others move toward the threatening person; and others scan for any additional threats and fan out to different parts of the venue.
For the agents who shield the candidate, "they're putting their body, their person, in front of that incoming threat," he said. "It's stressful. It's a lot of adrenaline."
For the agents who move in on the threatening person "the Secret Service trains to cover that person, so if they have something in their hand or on their body, you're going to feel it," said former special agent in charge Larry Johnson. "It's not to hurt the person, because they might just be very excited. But you want to get between them and the protectee, and cover their hands," he said.
This year's threats have ranged in seriousness, from attention-seekers, to protesters, to one individual who is accused of actually trying to harm a candidate.
"The Secret Service cannot determine the level of threat or danger until they actually get the person and interview them," said Johnson. For example, a stage-rusher will have their criminal record checked and be asked about their intentions, their feelings toward Clinton or Trump, and to assess whether they are suicidal.
Thomas DiMassimo, who tried to rush the stage on which Trump was speaking in Dayton on March 12, later told CNN, "I was thinking that I could get up on stage and take his podium away from him and take his mic away from him."
But Michael Sandford wanted "to kill Trump," police say, when he attempted to pull a police officer's gun from its holster during a Trump rally on June 18. Just the day before, Sandford had practiced shooting a gun for the first time, and he "further stated that if he were on the street tomorrow, he would try this again," according to a criminal complaint.
Candidates have tried to brush off the concerns, at least in public. Hillary Clinton continued her speech almost uninterrupted on Wednesday, even weaving in a remark about passionate advocacy while agents hurried across the stage to stop a protester. And Donald Trump, after the incident in Dayton, said later he would have been ready to go mano-a-mano had his protective detail not subdued the man.
"I was ready," Trump said, drawing roars of approval as he made a few fist-jabs. "I don't know if I would have done well, but I would have been out there fighting."