So Stephen Colbert turns out to be just as much of an a-hole as “Stephen Colbert.” Viewers have noticed.

After an initial burst of interest spurred by CBS’s big-bucks saturation advertising campaign for “The Late Show with Stephen Colbert,” the show’s ratings have tanked and it is now running third in late night behind NBC’s Jimmy Fallon and ABC’s Jimmy Kimmel, who rarely has managed second place before.

The reason? A survey by the Hollywood Reporter found that conservative viewers are turning off Colbert in droves. Nearly twice as many Republicans are watching Kimmel as Colbert, who has turned “Late Show” into a sort of wannabe MSNBC program.

The pattern is familiar: When a Democrat is the guest, Colbert is Barbara Walters. When a Republican is on, he turns into Tim Russert.

Gosh, whoever could have predicted that?

“Because comedy doesn’t work unless the underlying premise rings true, just about no conservative finds Colbert funny,” a Post columnist opined on April 11, 2014.

“So, though he will be dropping the faux-con shtick when he takes over Letterman’s chair, millions of conservatives won’t be watching. CBS is essentially writing off half the potential audience before the first show even airs.”

Ratings show that 47 percent of Colbert’s viewers identify as Democrats, 17 percent as Republicans. Kimmel’s audience is evenly split — 33 percent Democrats, 32 percent Republicans, while Fallon’s is nearly so (36 percent Democrats, 31 percent Republicans).

“Colbert Nation is filled with wealthy, socially liberal men who overwhelmingly support legalizing marijuana and want Bernie Sanders to be president,” pollster Jon Penn explained to The Hollywood Reporter. Some 30 percent of Colbert viewers report that they are atheists, which is the No. 1 “religion category” choice for “Late Show” viewers. (Memo to the US Marine Corps: Don’t bother advertising on Colbert. Manufacturers of pastel capri pants for men, on the other hand, should not miss out on this opportunity.)

Of course, CBS could spin this by saying that Colbert’s viewers are the sort desired by advertisers. Except in the first week of November, Kimmel not only beat “The Late Show” in total audience, he also beat him in the key 18-49 age group. Colbert isn’t targeting a demo; he’s pandering to a niche.

The pattern is familiar: When a Democrat is the guest, Colbert is Barbara Walters. When a Republican is on, he turns into Tim Russert.

There’s a tradition in comedy of taking it easy on someone who is gracious enough to play along. Even “Saturday Night Live” wasn’t harsh on Sarah Palin when she actually appeared on the show.

Colbert is different. Consider The Atlantic’s summary of his recent political interviews: “Even by Colbert’s standards, his interview with [Ted] Cruz featured much tougher treatment than any of his other political interviews to date . . . Colbert steered almost completely clear of politics in his moving interview with [Joe] Biden. And when Bernie Sanders appeared on the show last week, Colbert gave him space to deliver his campaign talking points with little interruption.”

Almost as if he was trying out a third personality — a parody of a know-nothing liberal pundit — Colbert made a complete ass of himself in front of Cruz by suggesting that the senator, being religious, necessarily equated his opponents with Satan.

“You’re a religious man, right, you’re a religious man? . . . What about your opponents politically, are they diabolical?” Colbert then attempted to argue the Constitution with Cruz, which is a bit like giving chess tips to Garry Kasparov. Cruz reminded the baffled comic of the existence of something called the Tenth Amendment, which reserves to the states those powers not specifically granted the federal government, and in so doing actually won a round of applause from Colbert’s audience.

Colbert is so unremittingly hostile to Republicans that he will shortly find conservative invitees declining to appear. (Except Ted Cruz, who would argue with a tree stump.) That means the Colbert show risks turning into an echo chamber in which viewers doze off as Colbert and his liberal guests beam lovingly at each other like a mother and child.

This can be a bit sick-making to watch. As The Atlantic put it, “Colbert said goodbye to Biden by practically pleading with him to run for president. With Cruz, there was no deadpan, and no praise. ‘I really appreciated you sharing your views with us,’ Colbert said, ‘and good luck with the campaign.’ ”

CBS had a far funnier, more talented and more interesting choice to replace David Letterman right in front of it: Craig Ferguson, former host of “The Late Late Show.”

As Andy Borowitz wrote in the New York Times of Ferguson’s stirring, surprising and funny memoir “American on Purpose,” “Almost every time Ferguson has a chance to go for a cheap, easy laugh — the mother’s milk of late-night comedy — he runs in the opposite direction. Take the opening scene in which he meets George W. Bush at a reception . . . He bonds with Bush as a fellow recovering alcoholic, clinking glasses of sparkling water with him as the president makes an earnest toast to America. I repeat: this is the opening scene of a book by a comedian. That’s what we in the comedy business call courage, and it pretty much sets the tone for the rest of this memoir, in which Ferguson admirably avoids wisecracks and instead goes for something like wisdom.”

Cheap shots vs. wisdom. CBS made its choice, and now it is paying the price.