“Honestly, my blood started boiling at that point,” she said in an interview on Monday, describing when the motorcade overtook her. “I saw somebody holding a newspaper; I assumed he was in there.”

Ms. Briskman explained that she was angry about steps taken by the Trump administration, including on health insurance and deportations. Earlier in the day she had learned from news reports at the nail salon that Mr. Trump was playing golf in the area.

“I just got angry,” Ms. Briskman said. “I lifted my arm and started flipping him off. I started thinking, You’re golfing again when there is so much going on right now.”

She kept her arm up until the rest of the fleet passed and then repeated the gesture a second time when the cars came to a stop and she caught up. She said she locked eyes with a man inside who she thought was a Secret Service agent, and then held the gaze of another man, but then the cars moved past her and drove away.

“I did not see him, but he was the target,” she said of Mr. Trump.

The moment might have faded into obscurity. But riding along with the president’s motorcade was a gaggle of journalists and photographers who captured the gesture in images that were distributed far and wide, spreading online.