David Letterman had been on late night television for 33 years, 22 of them hosting CBS’ “Late Show” before his last show ended May 20.

It seems a lot longer.

The wonder is that he lasted so long, billing himself as a comedian, which he clearly was not. He was so unfunny that he could never beat his arch rival, the nearly as unfunny Jay Leno.

During Leno’s monologue, he showed his uncertainty whether a joke was funny by dragging it out, as though he was afraid that what he said wasn’t funny.

Letterman relied on boom-bahs from Paul Shaffer’s band to help make his jokes appear funny. Shaffer’s value to the show wasn’t his banter with Letterman. It was his band’s amplification of the forced audience laughs.

One writer charitably wrote that Letterman left as an als0-ran, implying that this former weatherman and failed morning-show host never rose much higher.

It is likely that Letterman owed his continued presence on television to executive big-wigs who, for whatever reason, supported and paid for huge promotional advertising campaigns to persuade people to watch Letterman’s lame jokes and lamer interviews.

Remember that the show “Everybody Loves Raymond,” probably the most unfunny comedy series ever, remained high in the ratings because of incessant advertising, convincing audiences that it was a “must-watch.” Maybe nearly as much money was spent on advertising “Letterman” as was spent on “Raymond” convincing watchers that they were funny shows.

To be charitable to Letterman, he did do positive things to promote his own and his show’s popularity.

There was when, in October 2009, he emotionally confessed on the air to having been sexually involved with women on his staff. That brought remembrance of when Bill Clinton’s rise in popularity coincided with the number of philandering revelations.

Letterman’s revelation was almost as effective as when other celebrities confessed to having been abused as a child or having been addicted to drugs.

Then there was Letterman’s miraculous recovery from quintuple bypass surgery in January 2000. Who can forget the show in which he returned to the show after recovery and the love shown to him by the audience?

Shaffer and his band boom-bahed so much they had to take a day off afterward.

Maybe Letterman unintentionally revealed the reasons he acted the way he did on his show when he revealed in a recent New York Times interview that he has been taking an antidepressant in recent years.

Maybe that’s why he really did think he was funny.

However, Letterman may be right if he believes he had a more than casual influence on American politics. Given the mental capacities of his audiences, it is not difficult to believe that Letterman’s left-leaning jokes, his crass, vulgar criticism of anything and everyone conservative and his promotion of liberal guests have influenced the electorate.

Perhaps nothing illustrates the love felt for Letterman by the left than National Public Radio’s convoluted remark that Letterman closed out his show “on his own terms, guided by a subversive sense of humor that was severely allergic to sentiment or phoniness.”

Nobody, at least no one among us non-elite intellectuals, knows what NPR was saying, but the words look good and sound melodic.

There is reason to believe that Letterman’s replacement, already the darling of the left, will be every bit as funny as Letterman, if you call Letterman funny at all.