Washington (CNN) — Sen. Angus King said Thursday he will vote for Hillary Clinton in November after considering who would be the best decision maker during a nuclear attack.

"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.