The House January 6 committee will subpoena Ginni Thomas, the wife of the supreme court justice Clarence Thomas, if she will not testify voluntarily about her involvement in Donald Trump’s attempt to overturn the 2020 election.

“The committee is engaged with her counsel,” Liz Cheney, the panel vice-chair, told CNN’s State of the Union. “We certainly hope that she will agree to come in voluntarily but the committee is fully prepared to contemplate a subpoena if she does not.”

Thomas corresponded with Mark Meadows, Trump’s final chief of staff, and John Eastman, a law professor who shaped the congressional side of a push which culminated in the deadly attack on the Capitol.

She also corresponded with Arizona Republicans about attempts to overturn Joe Biden’s victory there.

Her activities have added to pressure on her husband. An arch-conservative on a court tilted firmly right, Clarence Thomas was the only justice to say Trump should not have to release records to the House committee. His wife’s communications with the Trump camp were subsequently revealed.

Some on the left have called for Thomas to be impeached – a political non-starter.

Cheney said: “I hope it doesn’t get to [a subpoena]. I hope [Ginni Thomas] will come in voluntarily. We’ve certainly spoken with numbers of people who are similarly situated in terms of the discussions that she was having … so it’s very important for us to speak with her.”

Steve Bannon, Trump’s former White House strategist, faces jail time after being convicted of criminal contempt of Congress, for ignoring a subpoena.

Meadows was also referred to the Department of Justice for failing to co-operate but the DoJ declined to pursue charges.

On Sunday Elaine Luria, the Virginia Democrat who presented the committee’s case in a hearing on Thursday, told NBC’s Meet the Press: “I don’t think [the DoJ] can revisit something that they’ve already dismissed but [Meadows is] certainly someone who has probably more information than anyone, you know, other than the folks who we have already heard from who were in the White House that day.”

She added: “If he’s listening, we’d love to hear from him.”

Cheney would not go as far as her fellow Republican on the January 6 committee, Adam Kinzinger, who has said he thinks the panel has proved Trump broke the law in his attempts to overturn the election.

She said: “I think that Donald Trump’s violation of his oath of office, the violation of the constitution that he engaged in, is the most serious misconduct of any president in the history of our nation.

“The committee has not decided yet whether or not we’ll make criminal referrals … I would also say that the Department of Justice certainly is very focused, based on what we see publicly, on what is the largest criminal investigation in American history.

“But there’s no doubt in my mind that the former president of the United States is unfit for further office.”

Luria echoed Kinzinger, telling NBC: “I sure as hell hope [the DoJ has] a criminal investigation at this point into Donald Trump.”

She also said Merrick Garland, the attorney general, “has already told us he’s listening, and if he’s watching today, I’d tell him he doesn’t need to wait on us because I think he has plenty to keep moving forward.”

The House committee has held nine public hearings, eight in a summer run which ended on Thursday with almost three hours on Trump’s inaction while his supporters attacked the Capitol.

There will be more hearings in September. Cheney said more interviews were scheduled and the committee “anticipate[s] talking to additional members of the president’s cabinet. We anticipate talking to additional members of his campaign.

“Certainly we’re very focused as well on the Secret Service and on interviewing additional members of the Secret Service and collecting additional information from them.”

The deletion of Secret Service text messages from 5 and 6 January 2021 despite an order from the committee to preserve them is another flashpoint in the continuing saga of Trump’s attempted coup.

On Sunday, Cheney repeated her praise of witnesses who have come forward, including Cassidy Hutchinson, a former aide to Meadows, and Sarah Matthews, a former White House press aide.

Asked about Republican attacks on such witnesses, Cheney said: “Certainly it is the case that the attacks against some of the women witnesses have been particularly vicious. I also think the response that we’ve seen from the House Republicans is really disgraceful.

“… I think our country is at a moment where we really have to all of us take a big step back and all of us say, ‘Look, the normal, sort of vitriolic, toxic partisanship has got to stop and we have to recognise what’s at stake.’ And … the leadership of the Republicans in the House need to be held accountable for their actions.”

Cheney, a stringent conservative, is nonetheless expected to lose her seat in Wyoming, over her opposition to Trump. She would not be fully drawn on whether she plans to run for the Republican presidential nomination in 2024, when Trump is expected to mount another campaign.

“I’ve not made a decision about 2024,” Cheney said, “and I am really very focused on the substance of what we have to do on the select committee, very focused on the work that I have to do to represent the people of Wyoming. And I’ll make a decision about 2024 down the road.

“But I do think as we look towards the next presidential election … I believe that our nation stands on the edge of an abyss. And I do believe that we all have to really think very seriously about the dangers we face and the threats we face and we have to elect serious candidates.”