She saw her mother at the Capitol melee and called her out — publicly.

For four years, Helena Duke, an 18-year-old high school senior in Massachusetts, had been growing further apart from her mother over their political views. She marched in protests for racial justice to her mother’s outspoken disapproval, she said. All the while her mother, a longtime Democrat, became ever more supportive of President Trump.

Then last week, a cousin of Ms. Duke’s sent her a video clip that had been going viral on social media. A Black woman is seen in the middle of a crowd of Trump supporters in Washington, D.C.; a white woman swings her hand at the Black woman’s face, the Black woman hits her in return; then the crowd angrily confronts the Black woman.

Ms. Duke immediately recognized the people at the center of the scene: her uncle, her aunt and her mother, who was the one who was hit. Her mother had never told her she was going to Washington, she said. Ms. Duke found all of it infuriating.

“I remember seeing the F.B.I. tweets saying that anyone who knows anything about the people that were at the Capitol or anything like to put their names out there,” she said. “After a lot of thought, I was like, this is really the right thing to do.”

So in a post to the “100 or 200” Twitter followers she had at the time, she wrote: “hi mom remember the time you told me I shouldn’t go to BLM protests bc they could get violent … this you?” and retweeted the video clip. In a follow-up, she added: “this is the liberal lesbian of the family who has been kicked out multiple times for her views,” and listed her mother, uncle and aunt by name.

In the days since the first tweet, which has been shared more than 80,000 times, Ms. Duke has achieved some degree of celebrity, raising thousands of dollars in a fund-raiser for her college tuition and hearing from strangers nationwide who feel alienated from their family members over politics.

What she has not done much is talk, beyond a few short texts, to her mother. Now living with her father, Ms. Duke said that even a short trip to pick up some clothes from her mother’s house over the weekend involved a police escort. She does not know exactly what her mother did in Washington, though her aunt’s name appears on a list of unrest-related arrests by the D.C. police, charged with simple assault. Neither Ms. Duke’s mother nor her aunt responded to messages seeking comment.

“It was hard for me, and I felt very, immense guilt for doing it at some point,” Ms. Duke said of posting the tweets. But she said, some of her cousins told her they supported her. “I really don’t think that I did anything wrong,” she said. “They should be held accountable.”