"I'm very concerned that Donald Trump will be graded on a curve. Just because he doesn't fly off the handle in the middle of this debate doesn't mean he's prepared to be president of the United States," Robby Mook said on CNN's "State of the Union."
He also slammed Trump for inviting Gennifer Flowers, a woman with whom former President Bill Clinton had an extramarital affair, to the debate, saying it's a further example of his bullying tactics.
"I think the fact that Donald Trump is spending the hours before the debate on this sort of thing is indicative of the kind of leader and the kind of president he would be," Mook said.
In a tweet earlier Saturday, Trump, the Republican presidential nominee, offered to bring Flowers to the debate, in response to the attendance of billionaire Mark Cuban, a vocal supporter of Democratic nominee Hillary Clinton.
The Trump campaign said Sunday that it did not expect Flowers to be a guest of the campaign.