The Republican race has gone nuclear.

One big reason why GOP presidential nominee Donald Trump is cratering lately — brand new polls show him 11 points down in Pennsylvania, 9 points down in Michigan, 15 points down in New Hampshire — is because people are catching on to the fact that he’s too unstable, too reckless and policy-ignorant, to command our nuclear arsenal in an unstable world.

That perception killed Barry Goldwater’s conservative candidacy in 1964 — perhaps unfairly, because veteran senator Goldwater was not a policy novice — and now it’s dogging Trump. Rightfully so.

The latest detonation came on the “Morning Joe” show, when co-host and ex-Republican congressman Joe Scarborough dropped this into our wakeup coffee: “Several months ago a foreign policy expert on the international level went to advise Donald Trump, and three times he asked about the use of nuclear weapons. Three times, he asked at one point, ‘If we have them why can’t we use them?’….Three times in an hour briefing, ‘Why can’t we use nuclear weapons?’”

Scarborough didn’t identify his source (was it the foreign policy expert? someone who’d spoken to the expert?); nor did he say whether he’s been sitting on this anecdote for months or just learned of it. But it rings true, because it jibes with everything else Trump has said.

Back in March, he told Chris Matthews that tactical nukes were a possible option during a European crisis (or during a confrontation with ISIS); in his words, “I’m not taking any cards off the table.” When Matthews said that our allies would be disturbed to hear a president talk so cavalierly about nukes, Trump replied, “Then why are we making them? Why do we make them?”

He doesn’t even know how our nuclear arsenal is set up. During a debate last December, he was asked to comment about the “nuclear triad” (three delivery systems – planes, subs, land-based missiles). He replied with a long riff about how “we need somebody absolutely that we can trust, who is totally responsible,” and then replied to a follow-up with, “I think — I think, for me, nuclear is just the power, the devastation is very important to me.”

He had no clue. He probably thinks triad is a synonym for a menage a trois.

Goldwater had logged years on Senate foreign policy committees. But after a string of comments about the alleged perils of signing a nuclear test-ban treaty and the alleged benefits of using tactical nukes in Vietnam, and some jokey talk about the mightiness of American missiles (we oughta “lob one into the men’s room at the Kremlin”), he got tagged as a dangerous loon. Lyndon Johnson’s Democratic operatives saw to that.

On the night of Sept. 7, NBC aired an LBJ campaign ad. It featured 3-year-old Monique Corzilius of Pine Beach, New Jersey (with her parents’ OK, for a fee of $100). She plucked some daisy petals and counted up to 10. Then came the stentorian voiceover, counting down to a nuclear launch. The bomb exploded as LBJ (quoting W.H. Auden) intoned, “These are the stakes — to make a world in which all of God’s children can live, or to go into the dark. We must either love each other. Or we must die.”

Hillary Clinton’s campaign hasn’t crafted anything as blatant as the famous “Daisy” ad, but it doesn’t need to. Its most ubiquitous ad — the one that shows little kids watching Trump on TV — connects with what the public is already sensing. And with what national security experts are darkly warning.

One such expert — Michael Hayden, who was George W. Bush’s CIA director — appeared Wednesday on “Morning Joe.” Scarborough’s much-circulated remark has regrettably overshadowed what Hayden said on camera: “(Trump) is inconsistent. And when you’re the head of a global super power, inconsistency, unpredictability, those are dangerous things. They frighten your friends and they tempt your enemies. And so I would be very, very concerned.”

Which well-credentialed national security experts are counseling Trump? Are there any advisers whom Hayden respects? Hayden’s reply: “No one.”

I trust there are many reasons why Trump is trailing Clinton nationally by 10 points — in the latest Fox News poll, no less. But it’s surely the nuclear factor that’s prompting many Republicans to question Trump’s candidacy. Too bad the Republican National Committee bylaws requires his willingness to quit. Too bad he’s deaf to something he said in that Republican debate last winter, because the irony is squirm-worthy:

“The biggest problem we have…is some maniac, having some madman go out and get a nuclear weapon.”

DICK POLMAN is the national political columnist at NewsWorks/WHYY in Philadelphia (newsworks.org/polman) and a “Writer in Residence” at the University of Pennsylvania. Email him at dickpolman7@gmail.com.