DOYLESTOWN, Pennsylvania—Richard Tems was about as loyal a Trump supporter as you can get. The Bucks County businessman and GOP committeeman has fiercely defended former President Donald Trump in editorials, interviews with reporters, and conversations with anyone within earshot. He voted for Trump twice and believed he was the right president at the right time. However, eight years after the New York businessman and entertainer came down the escalator in Trump Tower in June 2015, Tems said he would move on in next year’s Pennsylvania primary. He plans to vote instead for Governor Ron DeSantis, R-Fla., for the Republican nomination...