Earlier this month, the National Rifle Association was sued by the New York attorney general, who is seeking to dissolve the organization and said top executives had “looted” it. Now the N.R.A., which has faced two years of turmoil and infighting, is confronting a new adversary from within its own senior ranks.

Josh Powell, one of the group’s highest-ranking former executives, is poised to release “Inside the NRA: A Tell-All Account of Corruption, Greed and Paranoia Within the Most Powerful Political Group in America.” Mr. Powell, former chief of staff to Wayne LaPierre, the group’s longtime chief executive, says the N.R.A. is “rife with fraud and corruption” and writes that its finances “are in shambles,” and that “it has operated in the red for the past three years, despite annual revenues of roughly $350 million.”

The book, to be published Sept. 8 by Twelve, a division of Hachette Book Group, will arrive as Mr. LaPierre and the N.R.A. are fighting for their survival.

Both Mr. LaPierre and Mr. Powell, who for a time was seen as the N.R.A.’s de facto second-in-command, are named defendants in the suit brought by the attorney general, Letitia James, who has special jurisdiction because the N.R.A. was founded in New York. Beyond aiming to shut down the gun group, the suit seeks to remove Mr. LaPierre — who is accused of using millions of dollars of N.R.A. money to fund an extravagant lifestyle — and to bar both him and Mr. Powell from serving on nonprofit boards in the state.

Mr. Powell was assailed in the attorney general’s complaint, which said he was fired in December “for falsifying his travel expenses.” A lawyer for Mr. Powell has said that allegations in the suit “will be shown to be the result of false accusations made by Wayne LaPierre and others.” And Mr. Powell said he walked away from “a substantial severance package” because the N.R.A. wanted him to sign “a nondisclosure agreement, which I declined.”

Carolyn Meadows, the president of the N.R.A., said in a statement “It’s not surprising Mr. Powell would try to save his failing career by peddling fiction about the N.R.A. I doubt this is a book people will read, much less believe.”

Excerpts from Mr. Powell’s book provide an insider’s view into the N.R.A.

He writes about the day of the 2012 massacre at an elementary school in Newtown, Conn. — before he had even joined the N.R.A.’s leadership — and describes a conversation with Tony Makris, a senior executive at Ackerman McQueen, which was the N.R.A.’s longtime advertising firm until a bitter legal fight between the two organizations began last year.

“Get ready,” Mr. Powell writes that the executive told him. “This is going to be the mother of all gunfights. It’s really bad. There are still dead kids on the floor. Watch and learn. If we do this right the members will go nuts.”

Mr. Makris, in a statement through his lawyer, said, “A simple Google search will reveal all you need to know about Josh Powell.” Ackerman has previously said in court filings that its executives refused to work with Mr. Powell and accused him of sexually harassing one of its employees; a statement from Mr. Powell’s lawyer said no legal claim regarding harassment had ever been brought. (He was also once the subject of a sex discrimination complaint from an N.R.A. employee.)

After the Newtown massacre, Mr. Powell writes, “membership money and donations were an open spigot,” adding that “if we needed more, Wayne would just pour ‘gasoline on the fire,’ as he put it.”

The organization was working “to create and fuel the toxicity of the gun debate until it became outright explosive,” he writes. “We only knew one speed and one direction: Sell the fear.” He adds, “It worked to excite the most extreme faction of our membership — they ate it up.”

The book is the latest in a series of challenges for Mr. LaPierre, the most influential voice in the gun movement, who built the N.R.A. into a cultural and lobbying juggernaut. In the last couple years, he fought off an effort to oust him at the N.R.A.’s convention in the spring of 2019; clung to power for more than a year amid allegations of corruption; and pushed out the organization’s top lobbyist, its president, its advertising firm and several board members.

Such have been the N.R.A.’s travails that its longtime accounting firm, RSM, severed ties last year, people informed of the move said. RSM declined to comment.

Experts in New York charity law see Mr. LaPierre’s removal as all but a foregone conclusion, and say that without him at the helm, the N.R.A. has a better chance at avoiding dissolution. But the N.R.A.’s lawyers adamantly disagree, and a legal battle could take years. The organization’s sprawling 76-person board appears to be sticking behind Mr. LaPierre, and membership has surged.

“The board members that I’ve spoken to certainly continue to support Wayne; no one was surprised by this, and every board member I’ve spoken to is ready to fight,” Todd Rathner, a lobbyist from Arizona who is on the board, said in an interview.

He added that he had “heard that support from gun owners has been extraordinary,” as the N.R.A. moved swiftly to rally the faithful. An N.R.A. video posted on Twitter the day after the suit was filed, featuring video clips of Ms. James and scored with ominous-sounding music, urged viewers to “stand and fight” and become members. The group, which has more than five million members, added roughly 50,000 to its ranks in the week after the lawsuit was announced.

Ms. James, who called the N.R.A. a “terrorist organization” when she ran for attorney general in 2018, sent a new fund-raising email shortly after filing the complaint, promising to pursue “abuses of trust and unchecked greed run amok,” without specifically referring to the N.R.A.

But Mr. Powell writes that “the waste and dysfunction at the N.R.A. was staggering, costing the organization and its members hundreds of millions of dollars over the years.”

“What I witnessed during my time at the N.R.A. should horrify every gun owner and Second Amendment advocate,” he said in a statement. “Wayne La Pierre oversees his evil circus at the expense of all those Americans who are lawful gun owners and dues-paying N.R.A. members.”

Mr. Powell declined to discuss the attorney general’s lawsuit, which depicted him as a failed businessman with a trail of lawsuits, who had “routine disregard” for N.R.A. policies regarding expenses and contracts, and who was known for “abusive behavior towards N.R.A. and vendor staff.” Among the accusations leveled at him is that a consulting firm hired by the N.R.A. paid Mr. Powell’s wife $30,000 a month as an independent contractor, that he “took affirmative steps to hide the conflict,” and that he interceded to have his father hired as a photographer for N.R.A. events, according to the complaint.

A statement from Mr. Powell’s lawyer, provided earlier this month, said he had been “fired by Wayne LaPierre and the N.R.A.’s lawyers for his efforts to correct much of the misconduct cited in the attorney general’s complaint.”

The lawsuit seeks tens of millions of dollars in restitution from Mr. LaPierre and Mr. Powell, as well as from John Frazer, the N.R.A.’s general counsel, and Wilson Phillips, a former chief financial officer.

The depth of detail in the lawsuit is formidable. Mr. LaPierre is accused of using N.R.A. money to pay $13.5 million to a personal travel consultant over six and a half years; to cover $1 million in costs ferrying his family on private flights, without him; and for frequent trips to the Bahamas. There, he allegedly made use of an N.R.A. contractor’s lavish yacht called “Illusions.” He once spent more than $12,000 to put his niece up at a Four Seasons hotel for eight nights, according to the complaint.

“It’s inconceivable that he’ll be permitted to stay, and he’ll have a substantial amount of money to repay,” said Daniel Kurtz, a former chief of the charities bureau at the attorney general’s office, the division handling the case.

Sean Delany, another former chief of the bureau, said that if the attorney general “is able to prove even a fraction of the catalog of self-dealing and fraud that is recited in the complaint, it is impossible to see any court leaving LaPierre (or the other officers) in their positions.”

Shortly after the filing by Ms. James’s office, the N.R.A. hit back with a federal suit, claiming that the action brought by the attorney general was politically motivated and violated the gun group’s First Amendment rights.

“The so-called ‘experts’ know nothing about this case,” said Charles Cotton, the first vice president of the N.R.A., in a statement sent through the organization’s lawyers. “Many have been wrongfully predicting and, frankly, hoping for our demise for decades.” Mr. LaPierre, he added, is “not going anywhere.”