Arthur M. Schlesinger Jr., the Pulitzer Prize-winning historian and Kennedy insider who helped define mainstream liberalism during the Cold War and remained an eminent public thinker into the 21st century, has died, his son said. He was 89.

Schlesinger suffered a heart attack while dining out with family members Wednesday night in Manhattan, Stephen Schlesinger said. He was taken to New York Downtown Hospital, where he died.

Among the most famous historians of his time, Schlesinger was widely respected as learned and readable, with a panoramic vision of American culture and politics. He received a National Book Award for "Robert Kennedy and His Times" and both a National Book Award and a Pulitzer for "A Thousand Days," his memoir/chronicle of President Kennedy's administration. He also won a Pulitzer, in 1946, for "The Age of Jackson," his landmark chronicle of Andrew Jackson's administration.

With his bow ties and horn-rimmed glasses, Schlesinger seemed the very image of a reserved, tweedy scholar. But he was an assured member of the so-called Eastern elite, friendly with everyone from Mary McCarthy to Katherine Graham and enough of a sport to swim fully clothed in the pool of then-Attorney General Robert Kennedy.

He was a longtime confidant of the Kennedys, a fellow Harvard man who served in President Kennedy's administration and was often criticized for idealizing the family, especially for not mentioning the president's extramarital affairs.

"At no point in my experience did his preoccupation with women — apart from Caroline crawling around the Oval Office — interfere with his conduct of the public business," Schlesinger later wrote.

Liberalism declined in his lifetime to the point where politicians feared using the word, but Schlesinger's opinions remained liberal, and influential, whether old ones on the "imperial presidency," or newer ones on the Iraq war. For both historians and Democratic officials, he was a kind of professor emeritus, valued for his professional knowledge and for his personal past.

A native of Columbus, Ohio, and the son of a prominent historian, he was born Arthur Bancroft Schlesinger, Jr., but later gave himself his father's middle name, Meier. Family friends included James Thurber, historian Charles A. Beard and future Supreme Court justice Felix Frankfurter.

"My childhood was, in recollection, a generally sunny time," Schlesinger wrote in "A Life in the Twentieth Century," published in 2000. "I don't remember (or have repressed?) bad moments. There was an innocence about growing up in those days."

Schlesinger attended Phillips Exeter Academy and in 1938 graduated summa cum laude from Harvard University. During World War II, Schlesinger drafted some statements for President Roosevelt and served as an intelligence analyst for the Office of Strategic Services, a forerunner to the CIA.

Schlesinger emerged as a historian with "The Age of Jackson." Published in 1945, when he was just 27, the book offered a new, class-based interpretation of the Jackson administration, destroying the old myth that the country was once an egalitarian paradise. "The Age of Jackson" remained a major text despite eventual criticism — even by Schlesinger — for overlooking Jackson's appeasement of slavery and his harsh treatment of Indians.

Schlesinger was deeply involved with the Democratic Party, and even when writing about the past he minded the present. "The Age of Jackson," for instance, was completed during the presidency of Franklin Roosevelt and its characterization of President Jackson as a great 19th century populist was an acknowledged defense of Roosevelt and the New Deal.

Like many liberals of the 1940s, Schlesinger was also trying to reconcile support of the New Deal to the start of the Cold War. He responded by condemning both the far right and the far left, any system that denied the "perpetual tension" of a dynamic democracy. "World without conflict is the world of fantasy," he wrote in "The Age of Jackson."

In 1946, Schlesinger helped found Americans for Democratic Action, a leading organization of anti-communist liberals. Three years later, he published the influential "The Vital Center," which advocated a liberal domestic policy and anti-communist foreign policy. The book's title became a common political phrase, still in use decades later, and Schlesinger's call for defending American ideals abroad was endlessly revived as Democrats debated U.S. involvement in countries from Bosnia to Iraq.

In the 1950s, Schlesinger became increasingly involved in electoral politics, supporting Adlai Stevenson, the erudite Illinois governor and two-time loser to Dwight Eisenhower for the presidency. In 1960, the historian switched his loyalty to Kennedy, even as he acknowledged that Stevenson was a "much richer, more thoughtful, more creative person."