Patrick Caddell, the political pollster who helped send an obscure peanut farmer named Jimmy Carter to the White House, later became disillusioned with fellow Democrats and finally veered to advise supporters of Donald J. Trump, died on Saturday in Charleston, S.C. He was 68. His death, from complications of a stroke, was confirmed by a colleague, Prof. Kendra Stewart of the College of Charleston. While Mr. Caddell was considered instrumental in Mr. Carter’s victory, he also shared the blame for limiting him to a single term. He helped persuade the president to deliver a speech that was intended to inspirit...