It is easy for Alex Behr to gush about her son, Eli, whom she describes as a generous and thoughtful college junior who had a serious skateboarding phase. It is much harder for her to talk about his politics. Ms. Behr, 59, is a Democrat in Portland, Ore., who voted enthusiastically for Vice President Kamala Harris in the November election. She and her ex-husband were appalled that Eli, 20, decided to cast his first vote in a presidential election this fall for Donald J. Trump. When Eli brought a “Make America Great Again” hat home from college this summer, Ms....