Nancy Pelosi, chairwoman of the US House of Representatives, has appointed another Republican Trump critic to the committee to investigate the January 6 attack on the Capitol.
Congressman Adam Kinzinger said on Sunday that he had accepted Pelosi's offer.
This had already appointed the Republican Liz Cheney to the body.
Cheney is also a critic of Trump and was voted out of a leadership position in her parliamentary group under pressure.
Last week the dispute between Democrats and Republicans over the committee in the US House of Representatives had escalated.
The Republican minority leader of the Chamber of Congress, Kevin McCarthy, had proposed five Republicans for the body.
Pelosi had turned down two of them, who are considered particularly ardent Trump supporters.
McCarthy then withdrew all of his five nominees and accused Pelosi of "abuse of power".
The committee's first hearing is scheduled for Tuesday.
Kinzinger stated that he would attend this meeting.
The committee is to investigate the background to the attack on the Capitol.
Supporters of Trump stormed the seat of the US Congress in Washington on January 6th.
Five people were killed, including a police officer.