The main event began just as the undercard did -- with questions about Donald Trump. Tapper asked Carly Fiorina whether she would feel comfortable having Donald Trump's "finger on the nuclear codes."
Fiorina said that she believes Trump to be a "wonderful entertainer" whose "character will be revealed over time and under pressure." Tapper noted that that's not an answer to his question, and repeated it: "Would you feel comfortable with Donald Trump's finger on the nuclear codes?"
Fiorina dodged again, saying that's "for the voters to answer." Tapper turned to Trump, who began by saying "Rand Paul doesn't even belong on this stage, he's at 1 percent in the polls -- but anyway."
Tapper let Paul respond to the unprovoked insult, and the Kentucky senator said all he could do was laugh. "It was kind of a non sequitur," he said. Trump "was asked whether or not he'd be capable of, be in good hands in being charge of nuclear weapons, and he goes into this sideways attack on me."
"That goes to his character," Paul continued. "Do we want someone with this careless language to be negotiating with Iran? I think there's a sophomoric quality, entertaining quality to Mr. Trump. His visceral response to attack people based on their appearance -- tall, fat, ugly -- that happened in junior high."
Trump responded that "I've never attacked him on his looks, and believe me, there's plenty of subject matter there."
Watch the clip: