O’Donnell, right, on Capitol Hill with Sen. Daniel Patrick Moynihan in 1993, refers to himself as a “socialist.” (RAY LUSTIG/THE WASHINGTON POST)

Now here’s something you don’t see every day. Actually, you don’t see it any day: a cable-news host shredding his own employer for aiding and abetting a national “charade.”

“NBC has created a monster and it is called Donald Trump,” began Lawrence O’Donnell on his MSNBC show, “The Last Word,” last month. For nearly 15 minutes, O’Donnell hammered away, calling Trump “the most deranged egomaniac in the history of the NBC Entertainment division” and denouncing his would-be presidential run and “birther” allegations against President Obama as a “sleazy” publicity stunt for his NBC show, “The Celebrity Apprentice.” O’Donnell demanded that NBC executives disclose — “tonight, before you leave your offices” — whether Trump had already committed to another season of “Apprentice,” a disclosure that would have exposed his campaign as a sham.

Trump, of course, dropped out of the presidential race May 16, announcing his departure at an event during which NBC also announced that he would return for another season of his reality show.

And O’Donnell? After his very public broadside against his employer, he waited for a reaction. And waited. But there were no angry calls from NBC executives, no take-him-to-the-woodshed meetings at 30 Rock, MSNBC’s home. “We didn’t get a single call” from the brass, O’Donnell says, a sly smile breaking across his lips.

The non-reaction bespeaks either the network’s tolerance for self-embarrassment or O’Donnell’s critical importance to MSNBC. A frequent commentator and former fill-in host for MSNBC star Keith Olbermann, O’Donnell was awarded his own prime-time program at 10 p.m. on MSNBC only last September. Just four months later, after Olbermann’s stormy relationship with the network finally blew apart, MSNBC hustled O’Donnell into Olbermann’s plum 8 p.m. spot.

And just like that, O’Donnell became the leadoff man for MSNBC’s prime-time lineup of reliable libs and a key part — maybe the key part — of the network. Olbermann proved that the right host (and the right issue: opposition to the Iraq war) could draw in like-minded viewers and have a halo effect on the whole channel. Cable-news rival CNN has shown exactly the opposite: A weak program at 8 p.m. can be quicksand for the entire nightly slate.

Since his battlefield promotion, the 59-year-old O’Donnell has settled in. Though he can’t quite match Olbermann’s verbal pyrotechnics, he has connected with Olbermann’s audience. In the head-to-head battle between cable talkers, Fox News’s Bill O’Reilly out-draws O’Donnell by roughly 3 to 1 each night (just as he regularly trounced Olbermann). Nevertheless, O’Donnell has largely retained Olbermann’s million-plus nightly viewers, a tactical victory for MSNBC.

O’Donnell sounds almost as surprised as anyone by this turn of events. “I’m here for one simple reason,” he says after doing his show one night, his makeup still in place. “I subbed for Keith and the ratings did not go down. No one can explain to me how the ratings stayed the same.” He adds, knowingly: “This is entirely a luck business. William Goldman [the eminent screenwriter] said it best: ‘No one knows anything.’ ”

The daily ‘socialist’

While Olbermann was plainly left of center, O’Donnell has described himself on air as a “socialist.” He’s in favor of banning handguns (or more accurately, controlling ammunition sales). He’s against the death penalty, for raising taxes and in favor of universal government health care. “Liberals amuse me,” he said on MSNBC’s “Morning Joe” program after the election last fall. “. . . I live to the extreme left of you mere liberals.”

On air, O’Donnell can bring the heat — he has called O’Reilly “a joke” and “a liar” — but he’s also wary of engaging in too much mud wrestling. His discussions about politics with Olbermann holdovers such as Richard Wolffe and The Post’s Dana Milbank tend to be mostly sober and only mildly inflammatory.

O’Donnell also dabbles in lighter fare, such as Charlie Sheen’s looniness or Lindsay Lohan’s wayward ways. One of his segments on a program last week was a debate between Jon Stewart and O’Reilly about Obama’s decision to invite the rapper Common to the White House. He ran a clip of the discussion — lifted from “The O’Reilly Factor,” no less — that rambled for nearly four minutes. “At least I didn’t have to talk,” he quipped afterward.

O’Donnell says he’d like to do a more in-depth work, with a broader range of topics, but it’s not in the cards.

“If it were up to me, we’d be doing a PBS show,” he says. “I’m trying to train my show instincts to what works in this environment. There are things that deserve better conversations than we’re capable of having in eight-minute blocks. But a Jim Lehrer show or a Charlie Rose can’t happen here. Mostly, it’s me talking to people who agree with me . . . It’s not ‘Crossfire’ anymore. It turns out what works is op-ed TV, like the op-ed page of a newspaper [rather] than a debate.”

O’Donnell says he never planned to be on TV, let alone become one of its more prominent talking heads. But his varied career seems like perfect preparation for the gig.

After graduating from Harvard, he set out to be a writer. He scored a bestseller at age 31 with “Deadly Force,” an account of an unlawful police shooting in his native Boston in which his father, Lawrence Sr., acted as lead attorney for the victim’s family (O’Donnell himself provided legal research to his father and three brothers, all lawyers). After the book was made into a TV movie, O’Donnell went to Hollywood and tried writing there, though without much success.

When the 1988 Hollywood writers’ strike left him at loose ends, he accepted an offer to join the reelection campaign of New York Sen. Daniel Patrick Moynihan. The job turned into a staff position, and O’Donnell eventually became staff director of the Moynihan-chaired Senate environment and finance committees, where he wrote tax legislation, managed budget negotiations and worked on first lady Hillary Rodham Clinton’s failed health-care reform proposal.

He left Washington in 1995, but Washington didn’t entirely leave him. He began commenting on national politics, both in print and on TV. In 1999, Aaron Sorkin recruited him to help produce a dramatic series about the president and his staff called “The West Wing.”

O’Donnell spent seven years on the show, writing 16 episodes and variously serving as story editor, co-producer and executive producer. He broke off at one point in 2002 to produce and write “First Monday,” a short-lived CBS drama series about the Supreme Court starring James Garner. The next year, he created his own show, “Mister Sterling,” with Josh Brolin as a Mr.-Smith-goes-to-Washington-style senator. It lasted 10 episodes on NBC.

Married to the actress Kathryn Harrold, O’Donnell started acting, too, with bit parts in “Monk” and “The Practice” and a recurring role as a lawyer on HBO’s “Big Love.” Despite his day job, O’Donnell hasn’t completely sworn off fictional TV; he has a series under consideration at HBO, which he has described as a sitcom “about rich people in the age of [convicted con artist Bernie] Madoff.”

An evolving pundit

The pundit thing was a sideline for O’Donnell (he’s always been called Lawrence, to distinguish him from his father, known among family members as Larry). His showbiz background, Capitol Hill smarts and ruddy Irish American good looks made him a natural for occasional appearances on TV yakfests. NBC News’s former president, Andy Lack, spotted O’Donnell on one such appearance — “The Charlie Rose Show” in 1996 — and hired him as a contributor to NBC’s new cable-news network; O’Donnell’s debut appearance on MSNBC was during its first hour in 1996.

“I’ve never done anything that I need to be doing tomorrow,” he says. “If someone says, ‘We no longer want you here,’ I won’t show up again. It was true at ‘The West Wing.’ It was true in the Senate. I’ve lived a life of unplanned freelance employment. I don’t have some expectation” of continuity.

O’Donnell’s office in 30 Rock certainly looks like that of a man who doesn’t expect to stay long. Except for the flat-screen TV on a wall, there’s no adornment at all — just some books, a desk and a few hard chairs. From here you can almost see Fox News Channel’s offices a half-block away down Sixth Avenue.

O’Donnell’s work clothes — an array of dark suit jackets, ties and dress shirts — hang in a corner. Today, off-air, he’s wearing a long-sleeved blue T-shirt, casual black slacks and slip-on canvas sneakers. When 8 p.m. rolls around, he’ll put on the suit jacket and tie, but the bottom half of him (unseen on camera) will remain in slacks and sneaks.

“The Last Word’s” senior executive producer, Izabella “Izzy” Povich, says O’Donnell has quickly warmed to the faster pace of an 8 p.m. program after his brief run at 10 p.m. “I think he’s really coming into his own,“ says Povich, who was Olbermann’s producer (and is the niece by marriage of trash-talkmeister Maury Povich). “He’s a smart, sharp political analyst, because he’s lived it and worked it. What you’re seeing is him translating that into television.”

What matters in cable, Povich says, “is what gets the anchor ticking, what makes them emotional and wanting to fight. Any good cable show on TV has that common thread.”

O’Donnell can certainly get feisty and exude righteous vigor. On the few occasions when he’s had conservative guests, he’s tended to interrupt and argue. Watch his interview with Condoleezza Rice earlier this month for a vivid example. No doubt, as O’Donnell points out, Rice was stuck in auto-response mode, but O’Donnell barely let her respond, drawing several increasingly testy let-me-finish complaints from the usually composed former secretary of state.

Sometimes he goes even further. On the day Obama released his long-form birth certificate last month, O’Donnell had kooky birther Orly Taitz as a guest. O’Donnell tried to beat a mea culpa out of Taitz, repeatedly asking her if she accepted the “veracity” of the document. Taitz declined and instead tried to open a new line of attack on the president’s origins.

“Get her off this show!” O’Donnell sputtered in frustration. “Get her off. You’re fired. Go play with Donald Trump!” At which point Taitz disappeared from the screen.

It made for an electric confrontation, but it also raised a few questions, not least of which was: Why have on a widely discredited fringe figure like Taitz in the first place?

‘Shouting and tantrums’

“On paper, O’Donnell might seem to have more actual experience in Washington than a former ESPN shouter like Keith,” says Tim Graham, the director of media analysis for the conservative Media Research Center in Alexandria. “But he’s done some very weird shouting and tantrums.”

Graham says O’Donnell tends to find “racism around every Republican corner,” such as his denunciation of a recent Republican National Committee ad that he said contained “a racist message” because it suggested that white union bosses were telling a black president what to do.

O’Donnell has enough detachment and self-awareness to acknowledge that some of what gets on cable is amped up for effect, that part of it is an act and that, to a certain extent, he’s playing a character. Real life, and actual governing, he says, bears little resemblance to what you see on a talk show. “Working with Republicans was never like that,” he says of his time in government. People on opposite sides of the aisle, he says, tended to address each other reasonably, respectfully and usually honestly, even when they sharply disagreed.

“My weakness is I don’t take a lot of this [incendiary rhetoric] seriously,” he says after his show. “It’s hard to get me outraged. I hate the yelling stuff. I hate the way interruptions look.”

Really? So why does he do it himself at times? Where’s the integrity in that?

O’Donnell smiles to himself at the mention of integrity and tells a story about F. Scott Fitzgerald’s days as a Hollywood screenwriter. Struggling to complete his first script, Fitzgerald watched as a far-less-talented colleague produced one commercial success after another.

“I don’t understand it,” one of America’s greatest writers said to a studio boss. “I do everything you ask me to do and fail. He’s a success and he writes [bleep].”

“Ah,” responded the boss, “but even [bleep] has its own integrity.”