The text traffic also suggests that Ms. Thomas was in contact with Jared Kushner, the former president’s son-in-law and adviser. Sidney Powell, the lawyer advising Trump’s campaign team known for unleashing wild theories about voting fraud, comes up repeatedly. On Nov. 13, for instance, Mr. Trump included Ms. Powell in a tweeted list of his team’s lawyers. That same day, Ms. Thomas urged Mr. Meadows to support Ms. Powell, and said she had also reached out to “Jared” to do the same: “Just forwarded to yr gmail an email I sent Jared this am,” she wrote. “Sidney Powell & improved coordination now will help the cavalry come and Fraud exposed and America saved.”

When some of the president’s other lawyers began distancing themselves from Ms. Powell, Ms. Thomas warned Mr. Meadows not to “cave” to the “elites.”

In one text exchange right after the election, she tells Mr. Meadow that he needs to listen to Steve Pieczenik, a onetime State Department consultant who has appeared on Alex Jones’s Infowars to claim, among other things, that the Sandy Hook school massacre was a false-flag operation.

She also quoted language circulating on pro-Trump sites that said, “Biden crime family & ballot fraud co-conspirators (elected officials, bureaucrats, social media censorship mongers, fake stream media reporters, etc) are being arrested & detained for ballot fraud right now & over coming days, & will be living in barges off GITMO to face military tribunals for sedition.” She added: “I hope this is true.”

Ms. Thomas and Mr. Meadows have been like-minded associates for years, and she bestowed an award on him at a 2019 gathering of conservatives. While Ms. Thomas already had access to the president, White House aides said her influence increased after Mr. Trump named Mr. Meadows chief of staff in March 2020.

Mr. Meadows is no longer cooperating with the committee; a lawyer for Mr. Meadows, George J. Terwilliger III, did not immediately respond to requests for comment. Nor did Ms. Thomas or the Supreme Court. Mr. Terwilliger has argued that Mr. Meadows cooperated as much as he could without violating Mr. Trump’s assertions of executive privilege, and Mr. Meadows has filed suit against the panel to seek a court ruling to determine the validity of those assertions of executive privilege. Others challenging the committee’s subpoenas in court include John Eastman, a conservative lawyer and former clerk to Justice Thomas who wrote a memo arguing that Mr. Pence had the power to reject Electoral College votes for President Biden. Both cases could end up before the Supreme Court.