For 80 years, Li Rui, once a top aide to Mao Zedong, penned letters and diary notes detailing his long, topsy-turvy life near the center of the Communist Party. Before his death in 2019, Mr. Li’s daughter spirited the volumes out of China and into a California archive.

Now, “the Li materials” are the subject of a legal fight that pits Stanford University against Mr. Li’s widow in Beijing. It is a custody battle over an unofficial history of China.

In...

For 80 years, Li Rui, once a top aide to Mao Zedong, penned letters and diary notes detailing his long, topsy-turvy life near the center of the Communist Party. Before his death in 2019, Mr. Li’s daughter spirited the volumes out of China and into a California archive.

Now, “the Li materials” are the subject of a legal fight that pits Stanford University against Mr. Li’s widow in Beijing. It is a custody battle over an unofficial history of China.

In millions of handwritten Chinese characters, Mr. Li documents his early days in the party, the revolution that brought it to power and his experiences as secretary to Mao Zedong in the 1950s—plus the 20 years he spent incarcerated for maligning Mao’s economic policies.

Li Rui, in this 2004 photo in his study, was a rare insider willing to publicly criticize the Communist Party.

Photo: Li Nanyang/Associated Press

Mr. Li kept writing after he was politically rehabilitated and continued long into his retirement. A loyal communist cadre until his death at age 101, he was known among academics and journalists as the rare insider to publicly criticize the party over its Tiananmen Square crackdown, the construction of the Three Gorges Dam and, continually, the direction for China itself.

Mr. Li logged which officials he met and what they discussed as he climbed the party ranks, as well as the hour he rose from bed and the weather outside his sixth-floor Beijing balcony.

“Overcast to clear. Woke up a little bit past 6:30,” he begins an entry for a Sunday in October 2017 that presses glancing references to a former Chinese leader, the Cultural Revolution, a forest fire and commodity prices in between mentions of his TV watching, family photos and contact with a politically high-ranking family.

“Usually he just wrote the daily life facts, very seldom what he thought,” his daughter, Li Nanyang, told The Wall Street Journal. There was still drama, such as in his description of the June 4, 1989, Tiananmen crackdown: “Soldiers firing randomly with their machine guns, sometimes shooting the ground and sometimes shooting toward the sky,” he wrote.

“The whole day was unpeaceful, and I constantly wanted to wail. The party got things into an impasse, and there is no way to turn it back. I wonder how we can face the whole nation and apologize for the people. ”

— June 4, 1989

Li Rui’s diary for June 4, 1989, and the Tiananmen Square crackdown on pro-democracy activists.

Photo: Hoover Institution

In 2010 he wrote of Mao: “Mao completely went to the opposite direction of where the human race should be heading toward.” He also writes of meeting Xi Jinping in 1984 and 2002, as well as discussing Mr. Xi’s leadership in 2018 amid China’s efforts to remove presidential term limits, referencing a foreign news report headlined “Democracy is dead.”

Mr. Li was an honest and diligent scribe who, by championing the view that the party could reform itself, maintained relations across China’s political spectrum, said Chinese academic Feng Chongyi, now at Sydney’s University of Technology, who saw Mr. Li almost annually over the past decades.

“To keep that authentic record over that period of time, it’s simply valuable,” said Mr. Feng, who distinguished Mr. Li from some Chinese officials who have produced “diaries as propaganda.”

But publicly showcasing such an individualized version of core party events clashes with President Xi’s demands for a “correct outlook on history.” Chinese censorship has always been at its most intense over Communist Party matters and even the most mundane information is classified as a state secret.

Li Rui in February 2009 with his daughter Li Nanyang, who whisked most of his diaries out of China on a United Airlines flight to San Francisco in 2017.

Photo: Li Nanyang via Associated Press

In the rare instances when accounts from Chinese political figures have leaked, they have made a major splash, such as recordings snuck out of China from Zhao Ziyang, the deposed Tiananmen-era Chinese premier who spent the rest of his life under house arrest.

American journalist Adi Ignatius, who co-produced a 2009 Zhao memoir that shed fresh light on the crackdown and was based on the recordings, said that as the tapes were being smuggled out of Beijing his associates used an early version of encrypted email to avoid detection by state security. “It’s verboten to publish anything from behind the scenes,” said Mr. Ignatius, now editor of Harvard Business Review.

Mr. Li’s background makes his papers unique for anyone diving into his voluminous output. “Given the positions he held and the people he knew, I would expect them to be of great significance for research and helping understand the inner workings of Chinese elite politics,” said Anthony Saich, a China expert at Harvard Kennedy School, who also knew Mr. Li.

Publishing Mr. Li’s words has long been forbidden in China, Stanford argues in a court filing, so if his diaries had remained there they would “be suppressed and likely destroyed.”

“Mao’s act was totally against the universal values of freedom, democracy, progress in science, and the rule of law. Mao completely went to the opposite direction of where the human race should be heading toward. The ten years of the Cultural Revolution pushed China to the edge of total collapse. ”

— Jan. 9, 2010

But, were the papers stolen from China?

That is the question facing the U.S. District Court for Northern California, where lawyers on opposite sides of the family schism are volleying claims and counterclaims asserting ownership of the records.

In seeking to prove ownership, Stanford was joined in the Oakland case by the daughter, 71-year-old Ms. Li, against Mr. Li’s second wife and widow in Beijing, 91-year-old Zhang Yuzhen.

In a brief telephone call, Ms. Zhang declined to answer questions but said the diaries are an issue for the party’s Organization Department, where Mr. Li once held a post. China’s Foreign Ministry didn’t respond to a list of questions, including whether the government has backed Ms. Zhang’s case.

Her American lawyer, Matthew J. Jacobs of Vinson & Elkins LLP in San Francisco, said, “Our one and only client is Ms. Zhang, period.”

Ms. Zhang accuses her stepdaughter of exercising “undue influence” over Mr. Li, according to court files that allege Ms. Li “stole” personal information and “national treasures.”

A Beijing court in 2019 cited Chinese inheritance law in ruling that Ms. Zhang is entitled to the material.

Li Rui was accorded an official funeral with burial in Beijing’s storied Babaoshan Cemetery in 2019, where mourners outside gripped banners praising him.

Photo: greg baker/Agence France-Presse/Getty Images

Stanford’s attorney, Mark D. Litvack, of Pillsbury Winthrop Shaw Pittman LLP in Los Angeles, said Mr. Li makes it plain in the diaries themselves that he supported placing the material with Stanford’s Hoover Institution on War, Revolution and Peace. Mr. Litvack also said the Beijing court blocked Stanford from presenting evidence in that case, in particular its view that inheritance law isn’t relevant since the files weren’t in Mr. Li’s possession at the time of his death. “It’s an undeniable fact that we had them all when he was alive,” Mr. Litvack said.

“All events occurred in China, whether there was a theft or an inheritance or a gift, so under any scenario Chinese law applies, and a Chinese court has already ruled,” Mr. Jacobs said.

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For now, the 40 manuscript boxes and related digital files form collection No. 2019C100 in the extensive Chinese history archive at Hoover. They are open for viewing by appointment along with more than 6,000 Hoover collections from over 150 countries including World War I political posters, X-rays of Adolf Hitler’s head and decades of Afghan newspapers.

Ari Redbord, a former U.S. government lawyer with experience in international financial crime who isn’t involved in the case, said an American court is likely to focus on rightful ownership. “This sounds to me like a very classic property-law type case,” Mr. Redbord said.

Richard McGregor, the Australian author of “The Party: The Secret World of China’s Communist Rulers,” who visited Mr. Li at his home in 2003, said in the 2010 book, “He was perhaps the only senior insider living in China willing to talk publicly and in explicit detail about the taboo topic of Mao’s legacy.”

Despite often dissenting from the party line, Mr. Li was accorded an official funeral with burial in Beijing’s storied Babaoshan Cemetery. Top leaders paid their respects, including President Xi, who sent a wreath. Outside the funeral, a small group unfurled banners calling for democracy.

“At the end of the meeting we talked about how to evaluate Xi Jinping....[Mr. Xi’s father] Xi Zhongxun once instructed his son to follow two basic principles: seeking truths from facts, and being inclusive and generous to people. Xi Jinping’s response was that the former was difficult to achieve but the latter could definitely be done.”

— Jan. 16, 2012

What to do with Mr. Li’s prized diaries was a constant topic for family discussion, according to his daughter, who has written about her often-strained relationship with him. For years, she said, her father accepted his wife’s argument that they “belong to the party,” but nevertheless liked the idea of placing them with Hoover. Mr. Li had written of his visit to its archives in February 1989 and of being impressed that its China collection includes diaries of Chiang Kai-shek, who led the losing side in Mao’s communist revolution. (Stanford is also involved in long-running litigation over that material.)

By 2017, Mr. Li’s daughter had already delivered some of his letters to Hoover, which gave her a prestigious visiting fellowship position. For Ms. Li, the appointment was a “windfall” for taking the materials, Ms. Zhang argues in court filings; Stanford says the position was tied to Ms. Li’s assistance to archivists in deciphering her father’s sometimes hard-to-read scrawl.

In the Li home, resistance to sending the diaries overseas had quieted as President Xi shunted aside some of the old-guard officials who had long coddled the Li family, said his daughter.

On Jan. 30, 2017, Mr. Li wrote about speaking with his wife and daughter about the diaries and said “she”—though it is unclear whom this refers to—“agrees with my way to handle things, i.e. giving the diaries to the Hoover Institution for archiving.”

A few days later, Ms. Li said, “My father said go, go, go ahead.”

She stuffed decades of his diaries into two carry-on bags and nervously headed for a United Airlines flight to San Francisco, sweating in fear that customs authorities would confiscate her father’s files. “To this day, I don’t know why they didn’t check,” she said.

—Jim Oberman contributed to this article.

Write to James T. Areddy at james.areddy@wsj.com