For presidential candidates trying to hew to a carefully constructed message, taking questions at a town hall can be a bit dangerous.

Sen. Bernie Sanders (I-VT) found that out on Saturday, when a supporter of President Trump infiltrated one of his campaign events in Iowa.

As the man approached the microphone, Sanders sensed that danger, saying, “Uh oh, he’s looking at his phone. I’m in trouble.”

“Mr. Donald Trump, keep going, man. You’re doing a good job,” the man said.

As the crowd booed, he continued: “You know what, I’m a liberal.” More boos drown him out, but Sanders urged him to conitnue.

“I don’t agree with anything you say. I used to. I voted for you in 2016,” he said. “And I’ve been to Vietnam and seen what socialism has done. It’s destroyed the lives [of many].”

The crowd laughed, mocking him.

“Yeah, you can laugh all you want,” he said, “when they’re putting you in concentration camps.”

The scene got chaotic at that point, with several people shouting at the man and Sanders holding his hand up to try to regain control of the room.

But the man persevered.

“Donald Trump is helping our country. All right? He’s a good man… Socialism does not work.”

Then Sanders and the unidentified man shouted over each other, with Sanders saying, “Thank you for your contribution to the discussion.”

“Socialism doesn’t work! Socialism doesn’t work!” the man shouted. Police approached him as Sanders supporters yelled, “Get him out of here,” and he was escorted out of the room.

Earlier this month, Joe Biden blasted a man at another Iowa town hall, calling him a “damn liar” and challenging him to a push-up contest.

In that episode, the man began by telling Biden: “I’m not a Republican. I’ve got two problems with you. One, is you’re damn near as old as I am. You’re too old. I’m 83 and I know damn well I don’t have the mental faculties I did,” he said.

Then the man mentioned Biden’s son.

“But on the other hand, you sent your son over there to get a job and work for a gas company where he had no experience. … In order to get access for the president … you’re selling access to the president just like he was,” the man said.

Biden was furious. “You’re a damn liar, man. That’s not true and no one has ever said that.”

“Look, the reason I’m running is because I’ve been around a long time and I know more than most people and I can get things done,” Biden said. “And you want to check my shape? Let’s do push-ups together man, let’s run, let’s do whatever you wanna do.

“And number two,” Biden continued, “no one has said my son has done anything wrong and I did not, on any occasion.” The man interrupted, saying that he “never said” Biden was “doing anything wrong.”

“You said I set my son up to work at an oil company!” Biden yelled. “Isn’t that what you said? Get your words straight …”

The man in the audience replied: “I hear it on MSNBC.”

“You don’t hear that on MSNBC,” Biden said, approaching the man in a menacing way. “You did not hear that at all. I’m not gonna’ get into an argument here.”

The man shrugged and said: “You don’t have any more backbone than Trump does,” said the man. “I’m not voting for you.”

“Well,” Biden said, “I knew you weren’t voting for me, man. You think I thought you’d stand up and vote for me? You’re too old to vote for me.”