Sanders sensed trouble from the beginning when he saw the young man approach the microphone and said, "Oh, he's looking at his phone. I'm in trouble."
After being given the mic to ask Sanders a question, the man addressed the commander in chief directly, offering him words of encouragement.
"Mr. Trump, keep going man. You're doing a good job," he said. "You know what, I'm a liberal."
The man was then briefly cut off by boos from the crowd before Sanders urged them to let him finish. The man said he voted for Sanders during the 2016 Democratic primary but is now fully behind the president.
"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 mockingly laughed at his claims before he hit back, and reiterated the failures of socialism.
"You can laugh all you want," he shouted. "Donald Trump is helping our country. All right? He's a good man... Socialism does not work."
Bernie and the unidentified man continued to shout over one another until the man was escorted out of the room by what appeared to be law enforcement.
Fox News' Andrew Craft contributed to this report.