If you wonder whether Donald Trump’s lack of experience in the national security arena could hurt his candidacy, consider Senator Angus King, the only independent in Congress. After a recent trip on the “doomsday plane,” the one to be used by the president in the event of a nuclear attack, King has concluded that the presumptive Republican nominee is not fit to be commander-in-chief.

The Maine senator, a member of the Armed Services and Intelligence committees, said he and several other lawmakers recently flew in a mock exercise on the plane. That persuaded him that it would be too risky to put Trump in a position to order the use of a nuclear weapon. “In that situation, there is only one person making that decision,” King said Wednesday on the Charlie Rose PBS program. “One person has about 20 minutes to decide the fate of civilization.”

King said he’d worry both about Trump’s lack of strategic knowledge and his temperament, declaring, “he seems hot, impulsive” instead of “measured.”

The first-term senator caucuses with the Democrats but has endorsed candidates of both parties. As Congress’s only independent, he has nonpartisan credentials; the “doomsday” scenario is one that Hillary Clinton’s campaign may try to use against Trump in the general election.

Although King hasn’t officially endorsed anyone, he left no doubt he opposes Trump. The billionaire New Yorker’s lack of knowledge and reckless remarks have unnerved experts, including leading Republicans. During a Republican debate in December, for example, Trump was unable to explain what the term “nuclear triad” means — it’s the U.S. land, air and sea-based system for delivering nuclear weapons. He has also raised the possibility that Japan and South Korea might need nuclear weapons, contrary to decades of U.S. policy.

“Do we want two more countries with nuclear weapons and all those dangers?” King asked rhetorically. “It’s not a very thoughtful policy prescription.”

He said Trump’s call for NATO countries to pay more for their own defense is a familiar thought among U.S. policymakers, including President Barack Obama. But he said Trump has gone further, questioning the “underlying premise of NATO” and its relevance, which he called “an invitation to aggression.”

He added that Trump’s call for a temporary ban on Muslims entering the U.S. is “exactly” what the violent jihadist group Islamic State wants “to drive a wedge between the West and the peaceful Muslim community.”

King was generally complimentary of Clinton, whom he called “steady and thoughtful.” He said he doubted that her use of a private e-mail server while Secretary of State compromised national security. But he called it “a self-inflicted wound” that is “going to dog her.”

King said his unanswered questions about Clinton involve whom she would listen to as president, “and does she listen?”

He also said there’s “about 95-percent odds” that Obama’s Supreme Court choice, federal appeals court Judge Merrick Garland, would be confirmed in a lame-duck senate session if Clinton wins the presidency in November. King has previously criticized Republicans for refusing to hold a hearing on Garland’s nomination.

The Charlie Rose program aired on PBS Wednesday evening and is to be rebroadcast Thursday night on Bloomberg Television.

Albert Hunt is a Bloomberg columnist.