Sen. Angus KingAngus Stanley KingOvernight Energy: EPA watchdog slams agency chief after deputy fails to cooperate in probe | Justices wrestle with reach of Clean Water Act | Bipartisan Senate climate caucus grows Bipartisan Senate climate caucus grows by six members Senators fear Syria damage 'irreversible' after Esper, Milley briefing MORE (I-Maine) on Thursday endorsed Hillary ClintonHillary Diane Rodham Clinton3 ways government can help clean up Twitter Intelligence Democrat: Stop using 'quid pro quo' to describe Trump allegations The Memo: Bloomberg's 2020 moves draw ire from Democrats MORE for president.

“I’ve got to vote for Hillary Clinton,” the independent senator said on CNN’s “New Day”.

King said he couldn't back presumptive GOP nominee Donald TrumpDonald John TrumpThis week: House kicks off public phase of impeachment inquiry Impeachment week: Trump probe hits crucial point Judd Gregg: The big, big and bigger problem MORE.

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“It’s a question of judgment and temperament and this guy has not demonstrated to me the kind of coolness you need in that situation,” he said.

King said he made his decision after participating in a recent exercise on the aftermath of a nuclear conflict. King flew in the National Airborne Operations Center (NAOC) during the scenario, which he described as a “doomsday plane” for U.S. leadership in such an attack.

“We went up, took off across the country and then had a nuclear attack where an Air Force officer played the president and the secretary of Defense,” he said of the exercise.

“The first thing we noticed was a big clock,” King added. "The big clock said 28 minutes. That was the time the president had to make a decision.”

King said the experience rattled him and he could not envision Trump handling such a dire situation as president.

“What got me, it was almost physical, was in that situation there is only one person,” he said. “There’s no checks and balances, there’s no Congress, there’s no Supreme Court, there’s no consultation, there’s one person making a decision about the future of civilization.

"I just can’t in good conscience put somebody in that airplane whose coolness and sort of patience and judgment I have doubts about,” he added.

King also said Clinton showed grace under fire during her marathon hearing before the House Benghazi Committee in October.

“The other thing I thought about is Hillary Clinton sitting at the Benghazi hearing for something like 11 hours,” King added.

“That’s as good as you’re going to get of a photograph of someone making decisions under pressure."