His [Trump's] cavalier attitude about nuclear weapons is so deeply troubling. That is the number-one threat we face in the world. And it becomes particularly threatening if terrorists ever get their hands on any nuclear material. 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.

Presidential campaign (April 12, 2015 – 2016) - First presidential debate (September 26, 2016)