ADEL, Iowa —Joni Ernst had barely kicked off a town hall one morning here recently when the barrage of complaints about Donald Trump began. One after one, constituents took to the microphone to grill the first term Republican senator about the president — his attacks on John McCain, his emergency declaration at the border and why, in the opinion of several voters, Ernst hasn’t stood up to him more often. “Other senators are speaking up criticizing Trump and defending McCain, but I have yet to hear one word from you,” one man implored her. “Is there no line Trump can...