Sen. Angus KingAngus KingGovernment watchdog recommends creation of White House cyber director position Democrats step up hardball tactics as Supreme Court fight heats up Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' MORE (I-Maine) on Thursday endorsed Hillary ClintonHillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college 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 TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate 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."