A dozen potential Republican presidential candidates are about to sit down, one by one, with the biggest GOP donor in Iowa — a multimillionaire few people outside the state have ever heard of. Bruce Rastetter, an agribusiness mogul who’s made a fortune in pork, ethanol and farm real estate, has long worked behind the scenes to help bankroll conservatives across the country, but Saturday is a public coming out party of sorts for Rastetter as he hosts the first Iowa Agriculture Summit. It’s an event designed to promote farm policy in a state where pigs outnumber voters 10 to 1,...