Debbie Mucarsel-Powell knew that winning reelection in her swingy Florida district would be difficult. But it wasn’t until one night in February last year that the 50-year-old Democratic representative started to worry. That was the evening when then-presidential- candidate Bernie Sanders, in a 60 Minutes interview, showered praise on Cuba’s literacy programs under the Castro regime. “Is that a bad thing? Even though Fidel Castro did it?” the senator asked Anderson Cooper. Watching at home, Mucarsel-Powell was aghast. “How ignorant can you be?” she remembers thinking. “It was a complete insult to the Cuban diaspora that had fled that country.” Right away, she condemned Sanders’s remarks, but in her South Florida district, which is home to thousands of Cuban and other Latin American immigrants, the damage had been done. Republicans used Sanders’s comments to raise money for her opponent, Carlos Gimenez, and to paint Mucarsel-Powell as an ally of the...............