Rep. Liz Cheney said on Thursday that she does not believe the select committee created to investigate the causes of the Jan. 6 attack on the Capitol will be partisan, breaking even further from her party after accepting an appointment to the panel from House Speaker Nancy Pelosi. 

"It will be thorough. It will be professional. It will be serious and not partisan. I think this is something where we all have to come together to defend the democracy," Cheney, R-Wyo., said when asked about Republicans' concerns that it will be a partisan exercise with Democrats seeking to blame Republicans for the attack.

Cheney also sidestepped a question about whether former President Trump or House Minority Leader Kevin McCarthy, R-Calif., should testify in front of the committee. 

"I think the committee will decide the parameters of the investigation," Cheney said. 

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Rep. Bennie Thompson, D-Miss., who is the chairman of the select committee, also declined to rule out calling the former president or members of Congress before the committee. 

"We have to make sure that it never happens again," Thompson said. 

McCarthy made his displeasure with Cheney clear during his Thursday morning press conference, comparing her accepting a committee assignment from Pelosi, D-Calif., to members who have left their respective parties. 

"For somebody to accept committee assignments from Speaker Pelosi – that's unprecedented," McCarthy said. 

McCarthy did not answer whether he plans to appoint his own members to the Jan. 6 committee.

Pelosi, meanwhile, touted the fact Cheney agreed to serve on the investigative panel. 

"We're very honored and proud that she has agreed to serve on the committee," Pelosi said Thursday.

The other Democratic members Pelosi appointed to the committee are Reps. Zoe Lofgren, D-Calif.; Adam Schiff, D-Calif.; Pete Aguilar, D-Calif.; Stephanie Murphy, D-Fla.; Jamie Raskin, D-Md.; and Elaine Luria, D-Va. 

McCarthy is allowed to choose five members to Pelosi's eight. 

Rep. Liz Cheney, R-Wyo., speaks to reporters after House Republicans voted to oust her from her leadership post as chair of the House Republican Conference, at the Capitol in Washington, Wednesday, May 12, 2021. Cheney said Thursday that she believes the select committee to investigate the attack will not be partisan. 

Rep. Liz Cheney, R-Wyo., speaks to reporters after House Republicans voted to oust her from her leadership post as chair of the House Republican Conference, at the Capitol in Washington, Wednesday, May 12, 2021. Cheney said Thursday that she believes the select committee to investigate the attack will not be partisan.  (AP Photo/J. Scott Applewhite)

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Cheney's break from her party's leadership and its rank-and-file has been highly public and contentious. As many in the GOP sought to mend fences with former President Trump following the Jan. 6 attack on the Capitol by a mob of his supporters and his impeachment trial, Cheney continued to criticize the former president over his ongoing false claims that the presidential election was stolen. 

Cheney also insisted that the GOP needs to move on from Trump while most of the elected officials in her party aim to harness his support for the 2022 midterms. This led to Cheney being stripped of her role as the GOP conference chair, and she now faces several primary challengers back in Wyoming. 

The Jan. 6 committee is charged with investigating the circumstances surrounding the "domestic terrorist attack" and issuing a final report with recommendations for corrective measures. There's no timetable for the work to be completed, meaning the committee could keep the Jan. 6 attack in the headlines well into the 2022 midterm election year.

Fox News' Marisa Schultz contributed to this report.