For years, Donald Trump’s endorsement has functioned as the closest thing Republicans have to a political law of gravity. It has propelled challengers over incumbents, ended the careers of dissenters and, more often than not, settled primary contests before voters cast their ballots. But on Tuesday in Iowa, Trump's pick for governor lost to a first-time candidate who centered his campaign around fighting corporate agriculture and Big Pharma. The result exposed divisions within a Republican coalition that has often appeared remarkably unified under the President. Zach Lahn, a businessman and farmer, narrowly defeated Representative Randy Feenstra in the Republican primary, delivering one of the most significant setbacks to Trump’s political operation since his return to office. The loss was particularly striking because Feenstra had long been viewed as the race’s front-runner.…