Evan Barker believed she was a lifelong Democrat. Born into a Kansas City union family, she campaigned for Barack Obama while still in high school and was a community organizer for Hillary Clinton in 2015. Working for the consultancy Left Rising, she writes, “I built my name as a ruthless fundraiser … I was a live player who helped progressive insurgents” — including Alexandria Ocasio-Cortez, Cori Bush and Jasmine Crockett — “infiltrate from within.” But her new book, “Nothing Left” (Threshold Editions), out today, gives a look at how Barker lost faith in her party — and, in 2024, voted...