Indeed, the nation’s capital is in full spasm over Mark Leibovich’s cutting takedown of the city’s cozy culture in his new book, “This Town.” The fear: That it will send a chill through the elite after-hours social circuit — where the real business of this town often gets done between reporters and sources. What has rattled many is that Leibovich did a chunk of his reporting at parties and funerals at which he was considered a guest — or, at least, not a working journalist taking detailed notes.

For better or worse, there have long been some unwritten guidelines in Washington about what’s fair game — and what’s a cheap shot — in the coverage of social events. And unless you are officially covering a tony party with notebook visibly in hand, or camera visible, you’re expected to be, well, circumspect. Leibovich — or “Leibo” as he is called affectionately — is a self-admitted member of “The Club” and a popular New York Times reporter who is being privately assailed for exploiting his access to parties in order to skewer other members of The Club. The betrayal! Not since Truman Capote’s “Answered Prayers” knocked New York society on its heels with its thinly fictionalized revelations of real players who had thought the author was their friend has a book so riled a city’s upper echelons.

( PHOTOS: People in Mark Leibovich’s ‘The Town’)

“[Leibovich] must have made a judgment that it was worth it,” says one local businesswoman who has invited the reporter to events, “because I have to believe people will think twice before inviting him again.”

Former Reagan White House Social Secretary Gahl Burt contends that there are indeed “unwritten rules of decorum” in the capital, and frets that books like Leibovich’s have contributed to “a breakdown in loyalty and privacy.” She referenced a recent party at her place where a national political reporter was able to meet a White House aide who was refusing to cooperate on a profile. The off-the-record meeting, Burt said, allowed subject and reporter to connect in a low-pressure setting that ended up helping both.

For his part, Leibovich says that he is “always working as a journalist.” And furthermore, he adds, it’s a real circus out there.

“If you’re going to have a party filled with ostensibly public people, whether they are public because they’re elected to something or public because they are working all day to build their brand on Facebook or Twitter or whatever, I mean, I think like the whole notion of what is public and what is private has turned on its head, and that’s the era we’re living in,” he said.

( Also on POLITICO: Who’s up, who’s down in ‘This Town’)

Leibovich’s defenders agree that there is no such thing as a private event today — and hosts should always be on their toes.

“He’s been writing this book for a long time — and most people should have been aware of that,” said Sally Quinn, author and longtime chronicler of social Washington. “I knew when I invited him to my Christmas party he was working on it.

“Most journalists know where to draw the line,” adds Quinn. “I can’t tell you exactly where the line is. But it’s like pornography — you know it when you see it.”

Leibovich, in fact, wrote positively about the party given by Quinn and husband, former Washington Post Editor Ben Bradlee.

Others weren’t as lucky.

One of his prime targets in the book is Tammy Haddad, a former television producer who has hosted a lot of parties at which Leibovich was a regular. He portrays her as a social-climber and access-peddler, writing, “… perhaps a bit of a cartoon.”

And Leibovich clearly had a front-row seat to observe her. By his own count, he’s attended about 10 of Haddad’s parties over the years, including one or two of her famous annual garden brunches before the White House Correspondents’ Dinner. (He was dropped this year after Haddad got wind of what he was writing.)

Yet, he is merciless in describing her.

In one of the book’s more cringe-provoking scenes, Leibovich writes about a book party that Haddad hosted for the son of Susan Sher, Michelle Obama’s then-chief of staff.

“At one point, Tammy rushed over to me and the guy I was talking to and announced ‘ELIZABETH EDWARDS IS DYING! ELIZABETH EDWARDS IS DYING! I JUST GOT OFF THE PHONE WITH HER DAUGHTER! … Now, c’mon, come meet the novelist.’”

Said a mutual friend of the author and Haddad: “Mark just didn’t like the way she operates. I said to him, “Tammy has a role here. … She brings people together.”

Haddad declined to comment.

Beyond questions of social manners, there is a genuine concern that Leibovich’s reporting and tactics might pull the rug out from under one of the last venues for camaraderie in a polarized city: parties. As Henry Kissinger wrote of Washington in his 1979 book, “The White House Years”: “It is at their dinner parties and receptions that relationships are created without which the machinery of government would soon stalemate itself.”

“I think the damage here occurs if people avoid social settings that have historically been non-adversarial, where you can just sit around and have a beer and not worry that what you say will be reported,” says Mike McCurry, the White House spokesman under Bill Clinton. “A trust is broken and ultimately less information gets in front of the American people.”

Leibovich defends his reporting and ethics — and seems genuinely surprised by the criticism.

“I learn a lot at parties,” he told POLITICO in an interview from the lounge at the Algonquin Hotel in New York. “I think that that’s been a part of Washington reporting for a long time.”

He adds, “It’s always going to be a little uncomfortable to maybe reveal the secret handshake. … Unspoken rules are a little dangerous, especially in an age where public events are covered by reporters, and reporters are interviewing and doing videotapes of each other. So, you know, my feeling is if this book makes people within Washington uncomfortable, that’s fine.”

Leibovich is quick to add that he makes distinctions between large events that are covered by the media — even if he’s not covering them — and private gatherings, like a friend’s son’s bar mitzvah. “I mean, I consider those private events, and if I’m going to report on them, I’m going to ask special permission from the people who I’ll be talking about,” he said. “People, when they’re having private conversations with me know it, and I think that — I mean, I don’t cross lines there.” But a mammoth book party or the funeral of a prominent person where TV cameras are rolling are public events, Leibovich contends.

The reporter goes after some high-profile Washington targets in the book: NBC correspondent Andrea Mitchell; lawyer Robert Barnett; “Meet the Press” moderator David Gregory; GOP power broker Ken Duberstein; and Obama aide and confidant Valerie Jarrett. He also writes exhaustively about POLITICO, vacillating between calling it a journalistic phenomenon and promoter of shallow journalism. (To reinforce the intertwined nature of Washington, this might be a good place to mention that New York Times reporter Peter Baker, husband of POLITICO magazine editor Susan Glasser, is co-hosting his book party with six other prominent media figures from this town. Furthermore, I, too, have on occasion partied with Leibo in this town, and we were once colleagues at The Washington Post.)

In the elegant days of golden-lit dinners at Evangeline Bruce or Katharine Graham’s home — where presidents dined and serious players made private political deals — the news may have seeped into the public domain eventually. But it would have been unseemly, laughable, really, for Joe Alsop to write about what occurred in his column the next day.

In later years, the ground rules were clear: When working reporters were invited in, you were there to cover, you flashed your notebook and everything was fair game. But if you were not there specifically covering the event, it was assumed people could relax around you.

In Leibovich’s world — perhaps because of today’s 24/7 frantic news cycle — parties are the Wild West of journalism.

“Parties in D.C. have really become public spectacles because there are almost always media present,” said Susanna Quinn, who is married to lobbyist Jack Quinn, and who says she was “amused” that she is portrayed in the book as simply the pretty wife of a prominent person. “If you want something off the record, have a small dinner party where everyone knows the rules.”

There is a particular uneasiness that Leibovich chose to open the book with the pageantry and preening at Tim Russert’s 2008 memorial service — ridiculing some for displaying what he saw as overly dramatized grief over the untimely death of the host of the beloved “Meet the Press.” He was there as a guest and didn’t cover the event.

Although the actual service was broadcast nationally, what was happening in the seats was not. He describes Sen. Chuck Schumer (D-N.Y.), for example, as having his “head bowed, conspicuously biting his lips, squinting extra hard for full telegenic grief effect.” Duberstein, he writes, “keeps shaking hands and waving and looking mid-sentence over your glistening head to see who else is in the vicinity. He wears a big welcoming smile, which he relaxes, at the appropriate time into an expression of grave distress over the loss” of Russert.

“It’s a question of taste,” said McCurry, who once worked for Russert. “It’s one thing to tastefully record the event for posterity and quite another if you try to make people look foolish. A service is not the setting to ridicule.”

Russert’s widow, writer Maureen Orth and, their son, Luke Russert, declined comment.

Leibovich says he hasn’t noticed any change in his status in the Club so far. “I don’t expect to lose any real friends here, “ he says.

But he also knows well the old adage: In Washington, people aren’t friends; their jobs are friends.

“When people have an interest in talking to you, they’re going to talk to you. And because I’m attached to a large news organization, I think that probably will determine, you know, my status for a while,” he said.

“I don’t know if my party invitations have dried up. I don’t think I’ve gotten any in the last week or so, so maybe I am on some blacklist or something. But that doesn’t bother me at all. What’s important to me is good journalism … it’s being able to have the trust of sources but also the trust of readers.

“I haven’t violated any off-the-record things, ground rules. I mean, I don’t know what the unspoken codes are. Maybe that’s my problem.”