LITTLE ROCK, Ark. — While a student at the University of Arkansas in 1985, Sen. Mark Pryor (D) wrote his college thesis on the state of Arkansas’ two-party system. “The state’s Republicans have traditionally failed to produce politicians that Arkansas would elect,” he wrote. “The Democratic party, as a result, has thrived on a sort of perpetual motion.” That perpetual motion came to an end Tuesday night. Voters chose Republican Tom Cotton over Pryor, and for the first time in 141 years, there will be no Democrats in Arkansas’ congressional delegation. Republicans also won the gubernatorial race and every other...