DES MOINES, Iowa — As Democratic presidential candidates descended on the Iowa State Fair, a plane buzzed overhead, an ominous warning fluttering behind it on a banner: “Focus on Rural America.” Democrats hoping to win the White House in 2020 recognize how critical that advice is after 2016, when Hillary Clinton turned in strong performances in many cities and suburbs but lost rural voters 2-to-1, falling short to President Donald Trump by slim margins in Florida, Michigan, Pennsylvania and Wisconsin. Democrats clawed back some gains in rural counties in the 2018 midterm elections, and they want to build on that...