Footage shows a woman in a blue shirt bend down to yell at the girl, who was sitting in a stroller.
A Harris supporter pushes the woman away and appears to tell her not to yell at the toddler.
The exact cause of the confrontation remains unclear, but the child's father appeared to be holding a microphone, indicating he was addressing the crowd before the incident.
The video has gone viral and has gathered millions of views on social media.