“I did not want the morning to come where I will remove this bonnet and my wrap, and be met with more hair in the sink and an image in the mirror of a person who increasingly felt like a stranger to me,” Ms. Pressley said.

On the day last month when Ms. Pressley was to cast a vote to impeach President Trump, she realized she was completely bald. She wore a wig on the House floor because she felt she had to be present regardless of her hair loss, she said.

“I felt naked, exposed, vulnerable,” Ms. Pressley said. She also felt that she was participating in cultural betrayal, she said, because of all the young girls who looked up to her as a congresswoman who wore braids.

“I felt like I owed those little girls an explanation,” she said.

Toward the end of the video, the representative removes her wig and reveals a smooth scalp. After the video was released, her colleagues rallied in support on Twitter.

“Could you imagine losing all your hair on the eve of an enormously public day? And then turning that intensely intimate ordeal to make space for others? Ayanna, you are a living blessing,” Representative Alexandria Ocasio-Cortez, Democrat of New York, wrote on Twitter.

“Proud to count her as a friend, glad so many in the next generation have her as a role model,” former Senator John Kerry wrote.

“I am making peace with having alopecia,” Ms. Pressley said in the video. “I’m making progress every day.”