His death, from complications of a stroke, was confirmed by a colleague, professor Kendra Stewart of the College of Charleston.

While Caddell was considered instrumental in 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 the nation during an energy crisis and economic slump, but instead tarred 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.”

Ironically, Carter never mentioned the word “malaise” in his 1979 address to a nation fed up with endless lines at gas stations, inflation and joblessness. His solemn jeremiad bemoaning the “crisis of the American spirit” when “human identity is no longer defined by what one does, but by what one owns” was initially well received.

But it turned out that the crisis of confidence was in Carter’s ability to get the country going again.

In 1987, William Schneider, a political analyst with the American Enterprise Institute, called Caddell “the living American with the most direct experience in presidential campaigns, except for one, Richard Nixon.”

Patrick Hayward Caddell was born May 19, 1950, in Rock Hill, South Carolina. His mother was Janie (Burns) Caddell. His father, Newton P. Caddell, was a Coast Guard chief warrant officer. Patrick grew up near various Coast Guard stations, spending most of his youth in Falmouth, Massachusetts, and Jacksonville, Florida, where he attended parochial high school and also began taking his first public opinion surveys.

“Math was not my favorite subject,” he said, but at 16, for a mathematics project in his junior year, he fashioned a “voter election model” of the Jacksonville area for predicting elections based on early returns.

“I set up at the courthouse and called all the elections early with great abandon, with no idea what I was doing,” he said. “And they all turned out right.”

That was the extent of his formal academic training in public opinion research.

He was still an undergraduate at Harvard, where he earned a bachelor’s degree in political science in 1972 and started a thesis on “the changing South,” when he started polling professionally for Sen. George McGovern’s fledgling presidential primary operation.

McGovern, D-S.D., lost in a landslide; Caddell’s political acumen and polling expertise, translating data into tactical strategy, were among the few creditable outcomes of the campaign.

He established his own firm, Cambridge Survey Research, to conduct political campaigns. Although he spun off another company, Cambridge Reports, to advise corporate clients, he was criticized for capitalizing on his Washington connections — representing, among other clients, nuclear energy companies and the Saudi Arabian government — especially when, during the Carter administration, he became known as the president’s pollster.

Campaign staffs are not known for sharing credit, but in June 1976, when Carter had secured the Democratic nomination, his campaign manager, Hamilton Jordan, confidently told a reporter: “You know why Jimmy Carter is going to be president? Because of Pat Caddell — it’s all because of Pat Caddell.”

Jordan said that in helping Carter defeat Gov. George Wallace of Alabama in the Florida Democratic primary, Caddell had reaffirmed the campaign’s overall strategy but had also pinpointed where to concentrate its resources.

Before Carter was inaugurated in January 1977, Caddell advised him to stick to broad themes and issued a warning: “Too many good people,” he wrote in a 56-page memo, “have been beaten because they tried to substitute substance for style.”

By the late 1980s, Caddell had become disaffected with the Democratic Party, and appealing to alienated voters became a major element of the advice he delivered to candidates.

Scott Miller, a colleague who developed the political research site WeNeedSmith.com, said that many people would not miss his “needling and nettling,” but “the fact is, political calm made Pat very uneasy.”

“That calm means that the cement of the status quo is hardening, that the inevitable corruption of power is taking deeper root, that incumbent complacency is turning overripe on the vine,” Miller said in an email. “This drew him to the change leaders: to George McGovern, to Jimmy Carter, to Gary Hart, to Steven Jobs, to Ross Perot and eventually to Donald Trump.”

In 2016, Caddell became a frequent commentator on Fox News and advised Steve Bannon, who became Trump’s chief White House strategist, and Robert Mercer, a computer mogul and contributor to the Trump campaign.

Caddell, as a Harvard government major, understood that democracy and politics could sometimes conflict. He was quoted in “The Permanent Campaign” by Sidney Blumenthal as saying that Reagan’s tightly regulated contacts with the news media in his 1980 campaign set a model.

“I would never be involved in another presidential campaign where the candidate was openly accessible to the press,” Caddell said. “As a campaign operative, this makes it easier for me. As an American citizen, I shudder to think what people like me would do. It’s terrible for the country.”

Caddell is survived by a daughter, Heidi Caddell Eichelberger; three grandchildren; a brother, Daniel; and a sister, Patricia Roberts.