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

Sarah Matthews spent what would turn out to be her final day as a deputy White House press secretary working in a West Wing that President Donald J. Trump had essentially frozen in place as a riot engulfed the Capitol.

Ms. Matthews is one of the live witnesses who will explain what happened that day during a prime-time hearing on Thursday about the more than three hours on Jan. 6, 2021, during which Mr. Trump did nothing to quell the violence.

Ms. Matthews quit that day. But before she departed, she saw efforts by the White House press secretary, Kayleigh McEnany, to have Mr. Trump issue some kind of statement deterring his supporters, hundreds of whom had swarmed the Capitol as Joseph R. Biden Jr.’s Electoral College victory was being certified.

“I was honored to serve in the Trump administration and proud of the policies we enacted,” Ms. Matthews said in a statement that day. “As someone who worked in the halls of Congress, I was deeply disturbed by what I saw today. I’ll be stepping down from my role, effective immediately. Our nation needs a peaceful transfer of power.”

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.”