M. Stanton Evans was a key figure in guiding the modern conservative movement. (Patrick S. Korten)

M. Stanton Evans, an influential writer and a guiding force behind the modern conservative movement who helped promote the political career of Ronald Reagan and, in more recent years, sought to rehabilitate the reputation of Sen. Joseph R. McCarthy with a sympathetic biography, died March 3 at a nursing facility in Leesburg, Va. He was 80.

The cause was pancreatic cancer, said a longtime friend, Patrick Korten.

Mr. Evans was a leading theorist and polemicist of the right for more than 50 years. In 1960, he was a principal author of the Sharon Statement, the founding document of the Young Americans for Freedom and long considered a declaration of principles of modern conservatism.

The statement was so named because it was drafted at a meeting at the Sharon, Conn., home of National Review founder William F. Buckley Jr., a onetime friend of Mr. Evans’s.

The 368-word document defines much of the ideological framework of the conservative movement, including a belief in limited government and an equally fervent belief in an unlimited free market.

“When the government interferes with the work of the market economy,” one passage of the Sharon Statement reads, “it tends to reduce the moral physical strength of the nation, [and] when it takes from one to bestow on another, it diminishes the incentive of the first, the integrity of the second, and the moral autonomy of both.”

At 26, Mr. Evans became editor of the Indianapolis News, where he propounded his conservative views.

In 1961, he published the first of many books, “Revolt on the Campus,” in which he said a conservative intellectual movement was emerging to counter what he considered the prevailing left-wing views at many colleges.

For Mr. Evans, it was almost impossible to be too conservative or too hard on communism. In 1972, he led a movement to oust President Richard M. Nixon in Republican primaries on the grounds that Nixon was too liberal.

“I was never for Nixon until Watergate,” Mr. Evans quipped in the droll style that became his trademark.

He once dismissed the Kennedy administration and a popular dance craze of the 1960s in the same sentence: “I say the twist was originated in Washington by the Kennedy administration — a lot of frantic motion with no visible progress.”

In the 1976 presidential race, Mr. Evans had a key behind-the-scenes role in promoting Reagan, a former California governor, as a conservative alternative to President Gerald R. Ford. As president of the American Conservative Union at the time, Mr. Evans steered advertising money to North Carolina, where Reagan won the Republican primary.

Even though Reagan did not gain the nomination that year, a conservative tide moved the Republican Party to the right, culminating with Reagan’s election as president in 1980. Mr. Evans considered it the pinnacle of his influence.

Medford Stanton Evans was born July 20, 1934, in Kingsville, Tex., and grew up largely in Tennessee and Mount Rainier, Md., where his father worked on nuclear projects for the U.S. government.

At Yale, from which Mr. Evans graduated with honors in 1955, he became friends with Buckley, a founding father of the conservative movement. He briefly worked for Buckley’s National Review magazine before moving to Washington as managing editor of Human Events, a conservative weekly.

After leaving the Indianapolis News in 1974, Mr. Evans became a syndicated columnist and launched the National Journalism Center, a Washington-based training institute for journalists that he led until 2002. Mr. Evans taught at Troy University in Alabama before settling in Hamilton, Va. In recent years, he often addressed the Conservative Political Action Conference as an elder statesman.

His marriage to Sue Ellen Moore ended in divorce. He had no immediate survivors.

Among Mr. Evans’s books, none was more controversial than his 2007 biography of McCarthy, “Blacklisted by History.”

He argued that McCarthy, a Republican senator from Wisconsin, was correct to suspect a communist conspiracy at the highest levels of the federal government in the early 1950s.

He deemed McCarthy, who ruined many careers with his accusations, a patriot and a victim of a decades-long smear campaign — in effect, a left-wing form of McCarthyism.

The book was admired by some reviewers in right-of-center journals, but others found it wrongheaded and intellectually specious.

“Evans buys into the heart of the McCarthy conspiracy,” University of Texas historian David Oshinsky wrote in the New York Times, weaving a “remarkable fantasy, playing upon the deepest fears of right-wing Republicans.”

One of the harshest reviews came in the conservative National Review, where historian Ronald Radosh wrote that Mr. Evans sought to “exonerate McCarthy on virtually every count.”

Mr. Evans’s “own exaggerations and unwarranted leaps,” Radosh concluded, “parallel those made by McCarthy.”

Undaunted, Mr. Evans published another book along the same lines in 2012, “Stalin’s Secret Agents: The Subversion of Roosevelt’s Government.” He and co-author Herbert Romerstein charged that several high-ranking officials in the Franklin D. Roosevelt administration, and perhaps even first lady Eleanor Roosevelt, were communist sympathizers if not active Soviet agents.

Despite his hard-core conservatism, Mr. Evans thought of himself at heart as a journalist whose job was the relentless pursuit of facts.

“An information flow distorted from the right would be just as much a disservice as distortion from the left,” he said in a 1990 lecture at the Heritage Foundation. “What we really should be after . . . is accurate information. And I don’t see what any conservative or anybody else for that matter has to fear from accurate information.”