U.S. Sen. Angus King, a Maine independent, told CNN on Thursday that he’d vote for Democratic presumptive presidential nominee Hillary Clinton in November, questioning Republican Donald Trump’s “judgment and temperament.”

The former governor’s endorsement isn’t much of a surprise, since he caucuses with Democrats, generally supports their agenda and he has a preclusion to supporting politicians with deep governmental experience.

King, who sits on the Senate’s armed services and intelligence committees, told CNN’s Chris Cuomo that the endorsement was colored by a recent trip on the National Airborne Operations Center — the Air Force’s “doomsday” plane that would coordinate a government response to a nuclear attack.

When King got off the plane, his “knees were weak” at the realization that just one person, the president, would be “making a decision about the future of our civilization.”

Then, he said he thought about Trump, saying it’s “a question of judgment and temperament” and the bombastic billionaire with no political experience “has not demonstrated to me the kind of coolness that you’d need in that situation.”

King praised Clinton for her performance at a congressional hearing in October on her handling of a deadly 2012 terrorist attack in Benghazi, Libya that killed the U.S. ambassador and three other Americans, calling it “as good as you’re going to get as a photograph of somebody making decisions under pressure.”

“Putting those two things together and realizing the incredible importance of the presidency, … I’ve got to vote for Hillary Clinton,” he said.

King’s endorsement will always be notable because his backing in presidential contests isn’t always a given. He backed Republican George W. Bush in 2000 before endorsing Democrat John Kerry over Bush four years later. He has also backed politicians of all stripes since his election to the Senate in 2012, putting a premium on moderation.

In 2014, he endorsed Sen. Susan Collins, a Maine Republican, Sen. Jeanne Shaheen, a New Hampshire Democrat, Democratic 2nd Congressional District candidate Emily Cain and independent gubernatorial candidate Eliot Cutlerbefore a last-minute switch to Democrat Mike Michaud, who lost to Gov. Paul LePage.

King is the second member of Maine’s congressional delegation to back Clinton,after U.S. Rep. Chellie Pingree, a early-endorsing Democrat from Maine’s 1st District.

Collins and U.S. Rep. Bruce Poliquin, a Republican from the 2nd District, haven’t explicitly endorsed Trump, leaving LePage as Trump’s highest-ranking endorser in Maine. The governor has also said he’s considering challenging King for re-election in 2018.