"Is Donald Trump a serious candidate?" Paul asked during CNN's GOP presidential debate in Las Vegas, accusing Trump of wanting to close parts of the Internet and kill families of terrorists.
Paul continued, "It would defy every norm that is America, so when you ask yourself, whoever you are, if you think you're going to support Donald Trump, think: Do you believe in the Constitution?"
Trump fired back, saying he doesn't want to close parts of the Internet, and that his plan would entail using "our smartest and best to infiltrate their Internet so we know exactly where they go, exactly where they're going to be. I like that better."
Trump continued, to a mixed reaction from the crowd, "I just can't imagine somebody booing. These are people who want to kill us, folks, and you're objecting to us infiltrating their conversations. I don't think so."
Paul, who was added to the prime-time stage at the last minute after CNN tweaked its debate rules, was seeking a standout moment to break out of the crowded GOP field.
He was quick to attack Trump and other GOP rivals in his opening remarks.