Marco Rubio’s disastrous performance in Saturday’s debate came as no surprise to me. I had told friends for months that the senator had a glass jaw.

I saw Senator Rubio in October when he appeared at a meeting in New York to discuss the sharing economy. A fellow in the youngish, tech-savvy crowd launched into a lengthy off-topic attack under the guise of posing a question. Rubio froze. There was no security (the crowd was all invited), so the host of the event had to leave the podium and chase the interloper from the venue.

Every stand-up comic and any successful candidate knows how to deal with a heckler: you humor him, then belittle him and then bring the crowd to your cause. Or, if the situation is appropriate. you express righteous outrage.

Rubio couldn’t do that. Rubio’s only comment after order was restored was “Kinda weird.”

But Rubio’s jaw didn’t need Chris Christie’s attack or an audience heckler to be shattered.

The candidate himself, backed by Conservative Solutions PAC, has so attenuated his already thin national security resume that anyone paying attention knows he has a knockout coming in the general election.

After less than one term in the Senate, and with no military experience at all, Marco Rubio and his PAC have concocted a fiction that he is “an acknowledged national security ‘expert’”.

Intelligent observers rightly reply, “Says who?”

Rubio can readily mouth the “waging democracy as a benevolent hegemon” talking points preferred by the neoconservative wing of the Republican party. (He has even adopted the motif of their now-defunct “Project for a New American Century,” the think-tank that served as the Beltway cheerleaders for the Iraq War.) But there’s little evidence showing Rubio is “expert” in the field of national security, international relations, or diplomacy.

What “expertise” Rubio claims in national security is supported only by his modest five-year tenure on the Senate Foreign Relations Committee. One would have to considerably puff Rubio’s resume to say he even comes close to the giants of Republican Senate foreign and defense policy of the last generation or two. Comparisons to the likes of Richard Lugar (Oxford graduate, U.S. Navy intelligence officer who briefed Admiral Arleigh Burke), Barry Goldwater (Air Corps veteran; USAF Major General), or Charles Percy (Navy vet; CEO of the old Bell & Howell Company; University of Chicago grad) would be considerably overblown.

And that’s the problem. Rubio claiming “expertise” in a field so fraught with nuance and pitfalls puts him at risk of a knock-out by either the media or the ultimate Democrat nominee.

Those of us of a certain age remember Gerald Ford’s catastrophic debate against Jimmy Carter in 1976, when he insisted, “There is no Soviet domination of Eastern Europe” and then doubled-down on the comment when an incredulous moderator said, “I’m sorry, what?!” The flub arguably cost the president the election and cemented in the public mind an inaccurate, albeit widely repeated, notion that Gerry Ford was a dunce.

We also sat, dumbstruck, when Mitt Romney declared that “(Syria) is Iran’s route to the sea” in his debate in 2012. (We were thankful when Barack Obama’s own ignorance of the region’s geography allowed Romney’ flub to pass without what could have been a devastating correction from the president.)

Hillary Clinton may be many things, but she’s not stupid. And she has been briefed on and met many of the world’s leaders face-to-face, giving her a huge advantage in debate against a self-styled “expert” who has only crammed on flash cards. Even if Mrs. Clinton is indicted and the mantle of the Democrat nomination were to fall, as most assume, to Vice President Biden, Senator Rubio will still be at a considerable disadvantage. (Biden served over 30 years on the Senate Foreign Relations Committee, from 1975, before becoming vice-president.)

Old boxers know that if you cannot avoid being hit, it is best to lean into the punch, face down, to weaken the punch. But presidential candidates should know it’s better to avoid the blow completely by claiming expertise where none exists.

Rubio should drop the pretense.