Independent Sen. Angus King of Maine says he is supporting Hillary Clinton’s bid for the presidency.

King made the statement on CNN Thursday morning. He says he has greater trust of Clinton’s abilities in pressure situations, especially in foreign policy.

King told CNN that he fears Trump’s “judgment and temperament” aren’t suited for the White House, and that he lacks the “kind of coolness” needed for situations where leadership is really needed.

He said he decided to back Clinton after considering who would be the best decision maker during a nuclear attack.

He recounted a ride he took recently on the U.S. National Airborne Operations Center plane, more commonly known as the “Doomsday Plane,” the aircraft that would serve as the president’s command center during a nuclear crisis or other national emergency.

“I’ve got to vote for Hillary Clinton,” King 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.”

Maine’s other senator, Republican Susan Collins, has not made an endorsement since initially supporting Jeb Bush. She will be attending the Republican National Convention in Cleveland in three days.

King was elected to the senate in 2012. He caucuses with the Democratic Party and is a former governor of Maine.