WASHINGTON — Michael D. Cohen, President Trump’s onetime lawyer and fixer, released the foreword of his upcoming jailhouse tell-all on Thursday, posting to his website an introduction in which he promised stories involving the president and everything from “golden showers in a sex club in Vegas, to tax fraud, to deals with corrupt officials from the former Soviet Union.”

In the foreword to his memoir, “Disloyal: The True Story of the Former Personal Attorney to President Donald J. Trump,” Mr. Cohen claims that he had unique access to Mr. Trump, a man with “no true friends,” who trusted Mr. Cohen so much that his cellphone contacts were synced with his own.

“I bore witness to the real man, in strip clubs, shady business meetings, and in the unguarded moments when he revealed who he really was: a cheat, a liar, a fraud, a bully, a racist, a predator, a con man,” Mr. Cohen writes, claiming he has gained from those experiences a singular understanding of the president.

Mr. Cohen does not explain more in the foreword about his experience with Mr. Trump in Las Vegas, but the book will most likely revive questions about the veracity of an infamous claim in a dossier compiled by Christopher Steele, a former British spy, about Mr. Trump’s ties to Russia.

In it, Mr. Steele wrote that Mr. Trump had prostitutes urinate on a bed where President Barack Obama and his wife, Michelle, once slept, and that the Kremlin had recordings. Mr. Trump has denied the story, and it remains unsubstantiated. James B. Comey, who as F.B.I. director briefed the president about the dossier, has said that “it’s possible, but I don’t know.”

On Twitter, Mr. Cohen said he had “waited a long time to share my truth” as he posted a link to order a signed or an unsigned copy. In the foreword, he promises that “this is a book the president of the United States does not want you to read.”

Indeed, the government tried to stop the publication of Mr. Cohen’s book, according to a federal judge. Much of it was written on yellow legal pads by hand from Otisville Federal Prison.

Last month, a federal judge ruled that the decision to return Mr. Cohen to custody from home confinement amounted to retaliation by the government for his plans to publish the unflattering portrait of Mr. Trump. Mr. Cohen had been sent home because of the coronavirus pandemic. The judge ordered him to be released back into home confinement.

The book does not yet have a publication date. Mr. Cohen wrote online that it was “coming soon.” He has not said who his publisher is.

A White House spokesman, Brian Morgenstern, dismissed the book as “fan fiction,” and said that Mr. Cohen “readily admits to lying routinely but expects people to believe him now so that he can make money from book sales.”

Mr. Cohen writes in his foreword that “Trump had colluded with the Russians, but not in the sophisticated ways imagined by his detractors.” He claims that “Trump had cheated in the election, with Russian connivance, as you will discover in these pages, because doing anything — and I mean anything — to ‘win’ has always been his business model and way of life.”

Mr. Cohen presents himself as a repentant “bad guy” and says that while readers might conclude that they dislike him, they must be acquainted with seedy, venal characters like himself if they want to understand the world occupied by Mr. Trump.

“I stiffed contractors on his behalf, ripped off his business partners, lied to his wife Melania to hide his sexual infidelities, and bullied and screamed at anyone who threatened Trump’s path to power,” he writes.

Mr. Cohen says that as a presidential candidate, Mr. Trump “attempted to insinuate himself into the world of President Vladimir Putin and his coterie of corrupt billionaire oligarchs.”

“I know because I personally ran that deal and kept Trump and his children closely informed of all updates,” he adds.

Mr. Cohen claims authority on his subject, noting that “for more than a decade, I was Trump’s first call every morning and his last call every night.”

“I was in and out of Trump’s office on the 26th floor of the Trump Tower as many as 50 times a day, tending to his every demand,” he writes. Mr. Cohen claims that for many people trying to reach the former real estate developer, “when I spoke to them, it was as good as if they were talking directly to Trump.”

Mr. Cohen, 53, pleaded guilty in 2018 to campaign finance violations and other crimes stemming from a scheme to pay hush money to two women who said they had affairs with Mr. Trump before he was president. Mr. Trump has denied the accusations.

Mr. Cohen had been serving a three-year sentence at a minimum-security prison camp in Otisville, N.Y., about 75 miles northwest of New York City.

From prison, he writes, he has watched men like Rudolph W. Giuliani, William P. Barr, Jared Kushner and Mike Pompeo act as “Trump’s new wannabe fixers, sycophants willing to distort the truth and break the law in the service of the boss.” But he says none of them have been able to fill the void left where he once stood. “Trump doesn’t want to hear this, and he will certainly deny it, but he’s lost without his original bulldog lawyer Roy Cohn, or his other former pit bull and personal attorney, Michael Cohen.”

Mr. Cohen’s book is one of several tell-alls from former Trump insiders that are being released before Election Day. Stephanie Winston Wolkoff, a former friend and adviser to the first lady, Melania Trump, is set to publish “Melania and Me: The Rise and Fall of My Friendship with the First Lady” on Sept. 1.

Rick Gates, a former high-level aide on Mr. Trump’s 2016 campaign and a star witness in the Russia investigation, is expected to release a memoir, “Wicked Game,” in October.

Mr. Trump’s eldest son, Donald Trump Jr., is releasing a self-published book that is sure to paint a more flattering image of the president. He says he wrote the book, “Liberal Privilege,” in quarantine.