John Morton Blum, a Yale University historian who was one of the country’s foremost scholars of the presidency and who taught both candidates in the 2004 presidential election, died Oct. 17 at an assisted living facility near his home in North Branford, Conn. He was 90.

He had complications from pneumonia, said his son, Thomas Blum.

Dr. Blum became an academic phenomenon in 1954 with his second book, “The Republican Roosevelt,” which reassessed the reputation of Theodore Roosevelt, who had been dismissed by many historians as little more than a buffoon with a big grin. By sifting through Roosevelt’s papers, letters and decisions in office, Dr. Blum concluded that Roosevelt created a new, activist role for the presidency and help save the country from the runaway greed of business titans.

“Theodore Roosevelt changed the terms of American debate about public issues in two ways,” Dr. Blum told the New Orleans Times-Picayune in 1997. “The first way is important: Roosevelt stood for integrity and probity in personal and public affairs. Second, Roosevelt was the first modern American president to make a positive role for the federal government in containing private power and private wealth.”

Dr. Blum also wrote books about Woodrow Wilson and Franklin D. Roosevelt. He was considered, along with Arthur Schlesinger Jr., James MacGregor Burns and William E. Leuchtenburg, one of the most astute historians of the 20th-century presidency.

John M. Blum (Family photo)

President Bill Clinton once said Dr. Blum’s 1980 book, “The Progressive Presidents: Theodore Roosevelt, Woodrow Wilson, Franklin D. Roosevelt, Lyndon B. Johnson,” helped define his ambitions in the White House. For years, he taught a popular course at Yale on 20th-century politics.

When one of his former students, George W. Bush, was running for president in 2000, Dr. Blum confessed, “I­­­ haven’t the foggiest recollection of him.”

“But I remember Professor Blum,” Bush said during his 2001 commencement address at Yale. “And I still recall his dedication and high standards of learning.”

In 2004, when Bush faced Democrat John Kerry, both candidates had studied under Dr. Blum. Other students included Sen. Joseph I. Lieberman (I-Conn.), the Democratic vice presidential nominee in 2000, and Howard Dean, a former Vermont governor and Democratic National Committee chairman who sought the Democratic presidential nomination in 2004.

In his 1976 book, “V Was for Victory,” Dr. Blum examined the cultural and social life of America during World War II. Using elements of autobiography, he described how, as a Navy sailor at sea, he learned of the death of his boyhood hero, Franklin D. Roosevelt, by reading a Morse code message flashed from shore.

“They had hoped they had misunderstood, but they had not,” he wrote of his fellow sailors. “None of the boys or the young men on the ship really remembered any other president.”

John Morton Blum was born April 29, 1921, in New York City, the son of a businessman. In a 2004 autobiography, he wrote of how he had to overcome anti-Semitism at two bastions of WASP privilege, the private Phillips Academy in Andover, Mass., and Harvard University.

He graduated from Harvard in 1943 and returned after the war to receive master’s and doctoral degrees in history in 1947 and 1950, respectively. He taught at the Massachusetts Institute of Technology before joining the Yale faculty in 1957. He retired in 1991.

Survivors include his wife of 67 years, Pamela Zink Blum of North Branford; three children, Thomas Blum of Dobbs Ferry, N.Y., Pamela Blum of Kingston, N.Y., and Ann Blum of Arlington, Mass.; and three grandchildren.

Dr. Blum co-authored a high school history textbook, “The National Experience,” that has remained in print since 1963. In 2005, he published a mystery novel set on Yale’s campus, “An Old Blue Corpse.”

He often appeared on television documentaries, but his most unusual screen performance came in 1983 when he had a cameo in the Woody Allen film “Zelig,” discussing the fictional title character’s ubiquitous presence at major events of the 20th century.

“He wasn’t much of a moviegoer,” his son said Saturday. “It took him a while to realize Woody Allen was on the phone. He thought it was one of his students playing a prank.”