Sarah Matthews witnessed efforts by Trump aides to get him to act on Jan. 6.

She was previously interviewed by the committee and told it that a tweet Mr. Trump sent during the riot, in which he attacked Vice President Mike Pence as Mr. Pence was being taken to safety, was like “pouring gasoline on the fire.”

She is expected to describe discussions with other White House aides about what was happening as Mr. Trump sat, unmoving, taking in the violence on television.

Ms. Matthews spoke out recently in support of the committee’s star witness, Cassidy Hutchinson, a young legislative aide who worked for the White House chief of staff, Mark Meadows, and was a fly on the wall at a number of key moments. Ms. Hutchinson spoke at a hastily scheduled public hearing last month, during which she offered jaw-dropping new details about Mr. Trump’s behavior the day of the riot and about how Mr. Meadows appeared to be disengaged as it unfolded.

Amid efforts by Mr. Trump’s allies to tear down Ms. Hutchinson, Ms. Matthews said in a Twitter post, “Anyone downplaying Cassidy Hutchinson’s role or her access in the West Wing either doesn’t understand how the Trump WH worked or is attempting to discredit her because they’re scared of how damning this testimony is.”