At last, we know the reason why comedy writers don’t make fun of President Obama much.

It turns out the man is completely unmockable.

We learn this from Jim Downey, the longtime “Saturday Night Live” specialist in political japery. “If I had to describe Obama as a comedy project, I would say, ‘Degree of difficulty, 10 point 10,’” the writer says in the expanded new edition of the “SNL” oral history book, “Live from New York.”

“It’s like being a rock climber looking up at a thousand-foot-high face of solid obsidian, polished and oiled,” Downey says. “There’s not a single thing to grab onto — certainly not a flaw or hook that you can caricature. [Al] Gore had these ‘handles,’ so did Bush, and Sarah Palin, and even Hillary had them. But with Obama, it was the phenomenon — less about him and more about the effect he had on other people and the way he changed their behavior. So that’s the way I wrote him.”

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Got that? The charter Choom Ganger, confessed eater of dog and snorter of coke. The doofus who thinks the language spoken by Austrians is “Austrian,” that you pronounce the p in “corpsman” and that ATMs are the reason why job growth is sluggish. The egomaniac who gave the queen of England an iPod loaded with his own speeches and said he was better at everything than the people who work for him. The empty suit with so little real-world knowledge that he referred to his brief stint working for an ordinary profit-seeking company as time “behind enemy lines.” The phony who tells everyone he’s from Chicago, though he didn’t live there until his 20s, and lets you know that he’s talking to people he believes to be stupid by droppin’ his g’s. The world-saving Kal-El from a distant solar system who told us he’d heal the planet and cause the oceans to stop rising. The guy who shared a middle name with one of the most hated dictators on earth.

Nope, nothing there to mock. No way to get a grip on this polished, oiled obsidian. So comedy writers didn’t and mostly still don’t. Jim Downey isn’t in dereliction of his comedy duty to take down the high and the mighty, or so he begs you to think. He’s just too obtuse to see Scrooge McDuck quantities of comedy gold when they’re sitting right in front of him. Did Sarah Pallin lack for material to lampoon when she blasted away at Obama in her 2008 convention speech?

Downey, by the way, who admits to having been a “standard-issue Harvard-graduate commie” at the time he started out (with writing partner Al Franken) and now calls himself a conservative Democrat (so conservative he voted for Obama), is far-right by comedy-world terms. Former castmember Horatio Sanz dubbed him “the Karl Rove of ‘SNL’ ” because he occasionally took a poke at, say, the media for slavishly fawning over Obama during his primary battle with Hillary Clinton.

Yes, data nerds, there is empirical evidence that Barack Obama gets a free ride from comics.

In a new book, “Politics Is a Joke!” three academics tabulated 100,000 jokes told by late-night comics over the last 20 years. They found that in 2008 only 6% of the jokes were about Obama (Palin attracted nearly as many jokes in four months as a public figure as he did all year). And those jokes had a tendency to be about as barbed as cotton candy. Example cited by Tevi Troy in The Wall Street Journal: Jon Stewart said Obama visited Bethlehem so he could see “the manger where he was born.”

In every presidential campaign since 1992, the researchers found, comedians aimed more jokes at Republicans than they did at Democrats. Overall, twice as many barbs flew at the GOP.

“Our job is, whoever is in power, we’re opposed,” “SNL” chief Lorne Michaels told The New York Times in 2008. Agreed. And so they’ve been doing their job badly. Says who? Says . . . Downey.

Now that he has retired from the show and gained a little perspective, Downey comments in “Live from New York,” “I have to say, and even [Al] Franken agrees with me — I’ve talked to him about this — that the last couple seasons of the show were the only two in the show’s history where we were totally like every other comedy show: basically, an arm of the Hollywood Democratic establishment. . . . We just stopped doing anything which could even be misinterpreted as a criticism of Obama.”

Yeah, we noticed.