"The red MAGA hat is the new white hood," Milano, 46, tweeted Sunday.
"Without white boys being able to empathize with other people, humanity will continue to destroy itself. #FirstThoughtsWhenIWakeUp," she added.
Milano had previously posted a video of Nick Sandmann, a junior at Covington Catholic High School, and Native American Vietnam veteran Nathan Phillips.
Milano's post of the video came before a longer video emerged showing that Sandmann and his fellow students, who had participated in the March for Life earlier Friday, may have faced harassment of their own from other groups of protesters.
Sandmann, a junior at Covington Catholic High School in Kentucky, released a statement claiming he was targeted for harassment.