The former lawyer for the Oath Keepers militia pleaded guilty on Wednesday to advising members of the far-right group to illegally delete their text messages after the violent mob attack on the Capitol on Jan. 6, 2021.

At a hearing in Federal District Court in Washington, the lawyer, Kellye SoRelle, admitted to charges that included tampering with evidence and illegally entering and remaining in a restricted area of the Capitol grounds.

After Donald J. Trump lost the 2020 election, Ms. SoRelle, who is based in Texas, had close ties to the “Stop the Steal” movement, which claimed that Mr. Trump had been cheated out of a victory in his run against Joseph R. Biden Jr. She also served as the general counsel of the Oath Keepers and had a romantic relationship with the militia’s leader and founder, Stewart Rhodes, who was found guilty at a trial in Washington of seditious conspiracy for his role in the attack and sentenced to 18 years in prison.

During Mr. Rhodes’s trial, prosecutors presented evidence that he and Ms. SoRelle worked closely for weeks organizing the Oath Keepers to descend on Washington on Jan. 6. The evidence also showed that she was present at a mysterious meeting in an underground parking garage near the Capitol on the day before the attack where Mr. Rhodes met with Enrique Tarrio, the leader of the Proud Boys, another far-right group instrumental in the violence.

On Jan. 6 itself, Ms. SoRelle accompanied Mr. Rhodes to the Capitol, although neither entered the building. Still, court papers say that after the building was stormed and dozens of Oath Keepers came under scrutiny by federal investigators, Ms. SoRelle advised Mr. Rhodes and other members of the group to delete encrypted messages from their cellphones.

In the early days of investigation, Ms. SoRelle told reporters that she was cooperating with the Justice Department’s inquiry into the Oath Keepers’ role. She also spoke several times to staff investigators working with the House committee that investigated Jan. 6.

After she was charged, however, Ms. SoRelle sought to avoid trial by asserting that she was mentally incompetent. While the judge overseeing her case, Amit P. Mehta, initially agreed with her, he changed his mind after she received mental health treatment.

Ms. SoRelle emerged into the public eye one day after Election Day in 2020 when, as a member of a group called Lawyers for Trump, she raised false claims in a widely seen video that election workers in Detroit had committed voter fraud. Around the same time, prosecutors say, the Oath Keepers began to work for her as bodyguards.

By the following month, she had signed her name to two open letters to Mr. Trump that Mr. Rhodes had posted on the Oath Keepers website. The letters, introduced as evidence at Mr. Rhodes’s trial, called on Mr. Trump to take aggressive steps to remain in power. Those included invoking the Insurrection Act, a move Mr. Rhodes believed would have given Mr. Trump the authority to mobilize militias like his own to suppress the “coup” that was seeking to unseat him.

The trial also featured several text messages — some of them salacious — between Mr. Rhodes and Ms. SoRelle.

In one of the messages, on Dec. 29, 2020, Mr. Rhodes complained that he was getting tired of showing up at pro-Trump rallies in Washington where members of his group and other Trump supporters would simply “wave a sign, pray or yell.”

“They won’t fear us,” Mr. Rhodes wrote to Ms. SoRelle, “till we come with rifles in hand.”