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 right 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 in 1976, 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 the nation during an energy crisis and economic slump, but instead tarred Mr. Carter as a weakling who was unable to lift the country out of its malaise.

Instead, in 1980 voters chose Ronald Reagan, a Republican who promised a rosier vision that he would describe during his successful re-election campaign as “morning again in America.”