When Senator Mitt Romney, Republican of Utah, announced on Wednesday that he would not seek re-election in 2024, it was the culmination of a long estrangement between Mr. Romney and his party.

But Mr. Romney, the 2012 Republican presidential standard-bearer, made it clear he did not plan to quietly pack up his things and retreat from public life.

Instead, he appears to be exiting in a blaze of tea-spilling glory, choosing to share his unfiltered — and often unflattering — thoughts about his colleagues and his dismay about what has become of the modern-day G.O.P. by participating in a deeply reported biography, “Romney: A Reckoning.” The senator sat for hours of interviews with the author, McKay Coppins of The Atlantic, giving him access to emails, texts and his journals that Mr. Romney had been saving to potentially write a memoir. Realizing he could not be objective about himself, he said he chose to have a journalist write about him instead.

The result is the kind of tell-all, inside-Washington book that people have been hungry to read for years. The book is set to be published next month. On Wednesday, not long after Mr. Romney’s announcement, The Atlantic published an excerpt.

Here are six takeaways.

Mr. Romney lives a hermitlike existence in Washington.

Mr. Romney, 76, has few friends in Washington, and he did not follow President Harry S. Truman’s adage to get himself a dog. That, plus the absence of his wife, Ann, has left him living a lonely bachelor’s existence in a brick townhouse near the Capitol, where he spends most evenings stretched out in a leather recliner, eating dinner alone while watching shows including “Ted Lasso” and “Better Call Saul.”

Dinner is often a salmon fillet, courtesy of Senator Lisa Murkowski of Alaska, who gave him a freezer full of her home state’s specialty fish. Mr. Romney — who is partial to meatloaf bathed in ketchup and brown sugar — does not even like salmon, but he said he slathers it in ketchup, slaps it on a hamburger bun and makes do.

Almost every Republican shared his disgust with Trump.

Publicly, Mr. Romney has long been on an island in a party subsumed by Trumpism. Privately, he reveals, many of his colleagues, including Senator Mitch McConnell of Kentucky, the longtime Republican leader, are actually on the same page when it comes to his dim view of Donald J. Trump.

“Almost without exception,” Mr. Romney told Mr. Coppins, “they shared my view of the president.”

Mr. Romney kept a tally of his colleagues who approached him to privately express solidarity when he publicly criticized Mr. Trump, often saying they envied his ability to air his views. At one point, he told his staff, the list reached more than a dozen.

Mr. Romney also recalled a 2019 visit Mr. Trump made to the weekly Senate Republican lunch in the Capitol. The senators gave the president a standing ovation and were attentive and encouraging during his remarks about what he called the “Russia hoax.” They nodded when he said the G.O.P. would be known as “the party of health care” after they moved on from impeachment. But as soon as Mr. Trump left the room, the senators all burst out laughing.

He blasted Senators Josh Hawley and J.D. Vance, as well as former Vice President Mike Pence.

Mr. Romney, known for his genteel manners, could not hold back his deep disgust for some of his Republican colleagues.

“You’re the reason this is happening!” he recalled yelling at Mr. Hawley, Republican of Missouri, during the Jan. 6, 2021, attack on the Capitol. “You did this.” Mr. Hawley, who suggested that Mr. Trump could remain in office and that President Biden’s inauguration was not guaranteed, famously raised his fist that day outside the Capitol in a show of solidarity with the rioters.

Of Mr. Vance, the freshman Republican senator from Ohio, Mr. Romney said bluntly, “I don’t know that I can disrespect someone more.” After reading Mr. Vance’s best-selling memoir, “Hillbilly Elegy,” Mr. Romney said he had once been a fan. It made Mr. Vance’s swift conversion to a MAGA soldier all the more depressing to him.

“It’s not like you’re going to be famous and powerful because you became a United States senator. It’s like, really? You sell yourself so cheap?” Mr. Romney said. “How do you sit next to him at lunch?”

Mr. Romney, an observant Mormon, also had little regard for a fellow man of faith, Mr. Pence. No one, he said, had been “more loyal, more willing to smile when he saw absurdities, more willing to ascribe God’s will to things that were ungodly than Mike Pence.”

It all helped Mr. Romney come to a demoralizing conclusion a few months after the Jan. 6 attack. “A very large portion of my party really doesn’t believe in the Constitution,” he said.

Mr. Romney fantasized about challenging Mr. Trump in 2024.

Mr. Romney, who unsuccessfully sought the presidency twice, was tempted to make a third run in 2024, this time to mount a kind of anti-Trump, kamikaze mission possible only for a politician with nothing to lose.

“I must admit, I’d love being on the stage with Donald Trump … and just saying: ‘That’s stupid. Why are you saying that?’ ” Mr. Romney said. He dropped the idea once he realized the project would only help ensure another Trump victory.

He then toyed with forming a new political party with Senator Joe Manchin III, the centrist Democrat from West Virginia. His working slogan for it was “stop the stupid,” and he saw the goal not as running a likely doomed third-party candidate, but as endorsing “whichever party’s nominee isn’t stupid,” Mr. Romney explained. It is not clear if the plan has moved beyond the back-of-the-envelope stage.

Mr. Romney looks at the Senate as a strange curiosity.

Mr. Romney, a multimillionaire former businessman and governor, entered the Senate as something of a cultural anthropologist. At the Senate gym, he jotted notes in his journal about the odd sight of Richard Burr, the former Republican senator from North Carolina, walking on a treadmill in his suit pants and loafers.

Senators Sherrod Brown of Ohio and Richard J. Durbin of Illinois, both Democrats, were pedaling so slowly on their exercise bikes that Mr. Romney could not help but take a peek at their resistance settings. In his journal, he wrote: “Durbin was set to 1 and Brown to 8. :). My setting is 15 — not that I’m bragging.”

He also referred to the Senate as a “club for old men” and said he was struck by how his aging colleagues were driven by a deep-seated and apparently single-minded obsession with staying in power.

“Most of us have gone out and tried playing golf for a week, and it was like, ‘OK, I’m gonna kill myself,’ ” he said of the way many senators viewed a life without the influence that came with their elected offices.

Mr. Romney warned Mr. McConnell that their lives were in danger on Jan. 6.

In the days leading up to the riot, Mr. Romney received a warning about potential violence at the Capitol. He was concerned enough to text Mr. McConnell and share his fears.

“In case you have not heard this, I just got a call from Angus King, who said that he had spoken with a senior official at the Pentagon who reports that they are seeing very disturbing social media traffic regarding the protests planned on the 6th,” Mr. Romney wrote. “There are calls to burn down your home, Mitch; to smuggle guns into DC, and to storm the Capitol. I hope that sufficient security plans are in place, but I am concerned that the instigator — the President — is the one who commands the reinforcements the DC and Capitol police might require.”

Mr. McConnell never responded to the text.