Liz Cheney, the Republican vice-chair of the House January 6 committee, did “all she could” to protect the rightwing supreme court justice Clarence Thomas and his wife, the political activist Ginni Thomas, by blocking an in-depth investigation of Ginni’s involvement in Donald Trump’s attempt to overturn the 2020 election, a new book says.

In Stench: The Making of the Thomas Court and the Unmaking of America, the reporter and Democratic operative David Brock writes that “two Capitol Hill sources with personal knowledge” revealed a “dramatic truth, which might shock even some jaded Washington veterans not easily surprised by callow examples of power protecting power.

“Liz Cheney herself, the star of the hearings, doing her turn as independent-minded maverick Republican, did all she could behind the scenes to protect Ginni and Clarence Thomas and thwart the move to investigate further the implications of the Ginni Thomas texts to [Mark] Meadows,” Trump’s final White House chief of staff.

Thomas and Meadows swapped extensive messages about attempts to overturn Joe Biden’s election win. The texts, including indications Thomas discussed matters with her husband, were leaked to CNN in April 2022, as the January 6 committee continued its work.

The leak stoked uproar, prompting Norman Ornstein of the American Enterprise Institute, a conservative thinktank, to bemoan “a scandal of immense proportions”. As described by Ornstein, the texts showed “the wife of a supreme court justice is a radical insurrectionist [but] her husband has refused to recuse himself from any of the cases in which she has been deeply and actively involved.”

In his book, Brock argues that since his controversial confirmation in 1991, Clarence Thomas has helped shape a far-right court conclusively out of step with public opinion. Echoing prominent Democrats including the New York congresswoman Alexandria Ocasio-Cortez, Brock says Thomas should be impeached and removed.

Making his case, Brock outlines eight counts, from perjury in confirmation hearings, related to Thomas’s alleged sexual harassment of Anita Hill, to Thomas’s refusal to recuse himself from cases involving his wife and Trump’s election subversion. Prominent among such cases was one seeking the release of records connected to January 6, in which Thomas was the sole justice to say the records should stay under wraps.

In Brock’s words, the January 6 committee issued “no subpoena for Ginni Thomas, much less her husband” and sought “no public testimony, only an informational interview in which she was not under oath”.

Brock also reports that Denver Riggleman, a Republican congressman turned January 6 staffer who left the committee at the time of the text message leak and wrote his own book on the matter, says the “entire committee” worked to block a thorough investigation of Ginni Thomas. But Brock’s allegation that Cheney worked to shield the Thomases may prove explosive nonetheless, particularly as the former Wyoming congresswoman recently endorsed Kamala Harris, Trump’s Democratic opponent in the presidential election this year.

In Brock’s view, Cheney’s motivation to shield the Thomases was “in line with her own ambitions”, as she toyed with a presidential bid. Noting the documented friendship between the Thomases and Cheney’s parents – her father is the former vice-president Dick Cheney, who has also recently endorsed Harris – Brock says Liz Cheney deployed a “naked power move” connected to “raw political math”.

“No Republican candidate could survive coming out against the rightwing court-packing project,” Brock writes, referring to Clarence Thomas’s position as the most senior rightwinger on a court dominated 6-3 by conservatives, after the installation of three hardliners under Trump.

Cheney did not run for president. But as Brock notes, Ginni Thomas did avoid harsh scrutiny by the January 6 committee. In its 845-page report, her name does not appear even once. The committee did release a transcript of Thomas’s interview, in which she said she regretted texting Meadows and said: “You know, it was an emotional time. I’m sorry these texts exist.”

Cheney’s own book, Oath and Honor, was published last year. It includes a section on the leaking of the Meadows texts, which Cheney calls “unethical – and counterproductive for our investigation”.

She also describes an unnamed staffer’s “incredibly aggressive proposal … to subpoena all of Justice Thomas’s private communications in multiple forms of media”, based on a misidentified email address.

“This could have been a horrendous mistake,” Cheney writes. “It was lucky that we were being careful.”

Cheney writes that she has known Ginni Thomas “for decades” but “doubted very much that she was a mastermind” of Trump’s election subversion efforts. Noting Thomas’s critical public statements about her work for the January 6 committee, Cheney says she was “amused when liberals in the press or social media accused me of favoritism towards Ginni”, adding: “My view was that Ginni should be treated in the same way as every other witness who had engaged in similar conduct.”

Asked about Brock’s reporting, a source close to Cheney said it was inaccurate.

The source added: “Liz remains deeply troubled by Ginni Thomas’s apparent refusal to respect the rulings of our courts.”

Brock started out as a reporter for rightwing outlets, making his name with The Real Anita Hill, a book-length attack on Clarence Thomas’s accuser. But his other books include Blinded by the Right: The Conscience of an Ex-Conservative, in which he details a political conversion that has included founding political action committees and advising Hillary Clinton.

Speaking to the Guardian about his new book, which will be published in the US on Tuesday, Brock acknowledged Cheney’s decision to oppose Trump and how costly it proved.

“Liz Cheney obviously is a hero for many reasons,” he said. “But in this particular case, I think that her judgment was probably clouded by political and familial loyalties.

“There was definitely an effort in the committee to go further with the Ginni investigation. She obviously could have been subpoenaed. Clarence Thomas could have been subpoenaed. But they didn’t go down that route, largely because Liz Cheney was protecting the Thomases from that kind of scrutiny.”

• This story was updated on 15 September 2024, with further comment from a source close to Cheney