- "I just can't in good conscience put someone in that airplane whose coolness and patience and judgment I have doubts about," he said
- The Senate also praised Clinton's Benghazi panel performance
"I've got to vote for Hillary Clinton," the independent senator from Maine told CNN's Chris Cuomo on "New Day." "I just can't in good conscience put someone in that airplane whose coolness and patience and judgment I have doubts about."
King, the sole remaining senator who usually caucuses with Democrats who had not yet endorsed Clinton, said his support came after boarding the "Doomsday Plane," the airplane that would provide command and control in the case of a nuclear attack.
"When I got off the plane, my knees were a little weak with that realization how much power is in this one person. And then I thought about Donald Trump," he said. "And it's a question of judgment and temperament and this guy has not demonstrated to me the kind of coolness that you need in that situation."
King did not endorse any candidate during the presidential primary season and his backing of Clinton comes two days after the Senate's other independent senator who caucuses with the Democrats, Bernie Sanders, backed the presumptive Democratic nominee.
King also said Clinton's testimony before the Benghazi committee won him over.
"Never lost her temper. Never lost her cool," he said. "Never lost her patience. Answered every question. Was very solid. And I think most people even people who were opposed to her felt that was an amazing performance. That's as good as you're going to get as a photograph of somebody making decisions under pressure."