"Well, I have much better judgment than she does. There's no question about that. I also have a much better temperament than she has, you know?" Trump said.
The Republican nominee said Clinton has spent "hundreds of millions of dollars" on advertising aimed at him.
"I think my strongest asset, maybe by far, is my temperament. I have a winning temperament. I know how to win," he said.
Clinton rebutted by speaking about foreign policy, including the Iranian nuclear deal, and a comment Trump made earlier this month, stating he would blow Iranian ships out of the water if they made improper "gestures" toward American vessels.
"That is not the right temperament to be commander-in- chief, to be taunted," Clinton said.
Clinton then accused Trump of having a cavalier attitude toward nuclear weapons.
"So a man who can be provoked by a tweet should not have his fingers anywhere near the nuclear codes, as far as I think anyone with any sense about this should be concerned," Clinton said, using a line she has deployed in the past.
"That line's getting a little bit old, I must say," Trump shot back at Clinton.
"It's a good one, though," Clinton retorted.