Leo P. Ribuffo, a scholar of American political history who specialized in examining the rise of the far right, arguing that mainstream historians had underestimated and misunderstood its influence, died Nov. 27 at his home in Washington. He was 73.

The cause was hypertensive atherosclerotic cardiovascular disease, said a friend, Bruce Rich.

Dr. Ribuffo, a professor at George Washington University, was widely known among academic historians for his 1983 book, “The Old Christian Right: The Protestant Far Right From the Great Depression to the Cold War.” It won the Merle Curti Award from the Organization of American Historians for the best book of the year in American social or intellectual history.

Writing in the New York Times in 2017, historian and journalist Rick Perlstein called Dr. Ribuffo foremost among a handful of historians to counter long-prevailing views in academia about the far right and its appeal.

“Irascible, brilliant and deeply learned,” Perlstein wrote, “Ribuffo argued that America’s anti-liberal traditions were far more deeply rooted in the past, and far angrier than most historians would acknowledge, citing a long list of examples from ‘regional suspicions of various metropolitan centers and the snobs who lived there’ to ‘white racism institutionalized in slavery and segregation.’ ”

Dr. Ribuffo, wrote Perlstein, avoided the “psychologizing condescension” of describing far-right groups as “paranoid,” which was popularized in academic circles by Columbia University professor Richard Hofstadter in the 1950s and 1960s.

He told Perlstein that, after Donald Trump’s election in 2016, he would have written a response to disbelieving fellow citizens called “Why Is There So Much Scholarship on Conservatism and Why Has It Left the Historical Profession So Obtuse About Trumpism?”

Elizabeth Tandy Shermer, an associate history professor at Loyola University at Chicago, wrote in an online essay for the Society for U.S. Intellectual History that Dr. Ribuffo “never relegated . . . the Far Right to a paranoid few. He instead showed their ideas and organizational initiatives to be pivotal to understanding the entirety of American politics.

Leo Paul Ribuffo, whose father was a school janitor, was born in Paterson, N.J., on Sept. 23, 1945. He graduated from Rutgers University in New Brunswick, N.J., in 1966 and received a master’s degree in 1969 and a doctorate in 1976, both in history, from Yale University. He joined the George Washington University faculty in 1973.

“The Old Christian Right” grew out his doctoral thesis. The book focused on three agitators of the 1930s and the 1940s, William Dudley Pelley of the fascist underground Silver Legion, Gerald B. Winrod of the pro-Nazi Defenders of the Christian Faith and Gerald L.K. Smith, a clergyman who ran for president on the America First Party ticket. All three publicly acknowledged their racial bigotry and anti-Semitism.

At his death, Dr. Ribuffo was working on a social and cultural study of Jimmy Carter’s presidency. The planned book “is not in obvious ways related to ‘The Old Christian Right,’ ” Dr. Ribuffo told the reference guide Contemporary Authors. “But, as a historian, I must acknowledge continuity as well as change. My knowledge of religion, acquired while writing my first book, will help me to understand Jimmy Carter, the most devout Protestant president since William McKinley.”

Dr. Ribuffo’s marriage to Diana Rodriguez ended in divorce. Survivors include a half sister.