"Hillary apologized right away after that and said that she misspoke and that she regretted the comment. That's something that Donald Trump wouldn't do, you know," he told CNN's Jake Tapper during a special edition of CNN's "State of the Union," filmed at Harvard University's Institute of Politics in Cambridge, Massachusetts. The event, "War Stories: Inside Campaign 2016," also included Trump's campaign manager Kellyanne Conway and was the first time that the two managers appeared together alone since the presidential campaign.
"I think it definitely could have alienated some voters and that's why she got out there right away," Mook added.
Clinton sparked controversy at a September fundraiser in New York when she maligned "half" of Trump's supporters, going as far as to say that they are "irredeemable."
"To just be grossly generalistic, you can put half of Trump supporters into what I call the basket of deplorables," Clinton said. "Right? Racist, sexist, homophobic, xenophobic, Islamophobic, you name it."
On a special assignment from the Clinton campaign, Diane Hessan studied how undecided voters were responding to the campaign.
She wrote an op-ed in the Boston Globe sharing reflections from her study, which showed the reaction to the "deplorables" was stronger than when FBI Director James Comey sent a letter to Congress saying they were probing to see if additional emails on the laptop of one her top aides could have an impact on a closed investigation to Clinton's use of a primary email server.
Though the FBI eventually determined the emails did not change Comey's mind on his decision not to pursue criminal charges, members of Clinton's campaign have repeatedly pointed to Comey's letter and its proximity to Election Day as the defining moment that hurt the Democratic presidential nominee's chances, a point those staffers made repeatedly at this week's events.
"We were expecting to perform better with suburban women in particular. We saw those numbers a lot stronger than what happened on Election Day. We do think that was because of the Comey letter," Mook said.
He continued, "We saw a lot of young people go to third party candidates, we think the letter had a lot to do with that as well. So there were a number of reasons for this. But lead among them, in my view, would be that letter from Director Comey."
Hessan's op-ed said the "deplorables" moment appeared to have had more than an impact than Comey did.
"There was one moment when I saw more undecided voters shift to Trump than any other, when it all changed, when voters began to speak differently about their choice," she wrote. "It wasn't FBI Director James Comey, Part One or Part Two; it wasn't Benghazi or the e-mails or Bill Clinton's visit with Attorney General Loretta Lynch on the tarmac. No, the conversation shifted the most during the weekend of Sept. 9, after Clinton said, 'You can put half of Trump supporters into what I call the basket of deplorables.'"
"All hell broke lose," she added.
Conway pointed out that Clinton didn't actually apologize for the use of the term "deplorables" as Mook described.
"She didn't say that really. She said she regretted putting a number on it," she said.
Mook conceded, "She regretted her choice of words, but Donald Trump never apologized."
"Look, you're talking about one instance where Hillary Clinton said one thing. She immediately explained that she regretted," Mook added.
Conway responded, "I think she regretted getting caught," she said.