Chapin, who lived in New York City with his wife, Diana, died of a heart attack at the Chapin family home in Andover, N.J., where they frequently spent time.
Born into a prominent family -- his brother was the late singer Harry Chapin, his father James is a noted jazz drummer and his forebears were among New York City's elite -- Chapin carved his career from an interest in history, a phenomenal memory and a gift for sharp insights on current events.
From 1994 to 2000, he was senior adviser to Mark Green, New York City public advocate. Earlier, he taught history at Yale and Rutgers universities.
He also served on the boards of the Queens Borough Public Library and a hunger charity founded by his brother.
"I've never seen anyone with such an encyclopedic mind matched with such a big heart," said George Stephanopoulos, host of ABC-TV's "This Week" with whom Chapin carried on a lively e-mail exchange of ideas and opinions. "This is a big loss."
A cheerful and resourceful ideological warrior, Chapin was widely read and admired across the political spectrum.
"Jim Chapin was the smartest person I ever met -- and the most generous with his time and intellect of anybody I've known," said Jim Pinkerton, a former top aide to President George H. W. Bush, columnist for Newsday and contributor to Fox News.
Looking at events from the left for UPI, Chapin analyzed political issues, candidates and trends and debated issues with fellow UPI political analyst Peter Roff, taking the liberal position while Roff debated from the conservative point of view.
"Jim was actually hired to be a political analyst writing from a liberal-left perspective, which he did," said UPI Editor in Chief John O'Sullivan, who hired Chapin in 2000. "But he managed to make his analyses palatable and persuasive to people right across the political perspective.
"That's a very rare gift. It may have had something to do with the fact that he was one of the nicest people in the world."
Possessed of nearly total recall of historical events and people, Chapin was able to relate them to the latest political trends.
Writing in early September, Chapin said the lessons of history were being missed a year after the Sept. 11, 2001, terror attacks:
"The United States -- leaders and people together -- is not taking the war on terror seriously," Chapin wrote.
"Is America at war or at peace? There's a simple way to tell -- in a genuine war, foreign policy considerations override domestic ones -- in a 'phony war,' foreign policy is held hostage by domestic concerns.
"LBJ's Vietnam disaster was caused, in part, by his unwillingness to sacrifice his domestic priorities by admitting how much the war was going to cost. FDR's success in World War II was achieved in large part because he abandoned 'Dr. New Deal' for 'Doctor Win the War,' and recruited many of his leading foes into his administration.
"So far it's not hard to tell which way the Bush administration is going on the choice between being like FDR or being like LBJ. It's all the way with LBJ," he concluded.
Chapin held a doctoral degree in history from Cornell University, and had taught history at Yale University, and diplomatic and military history at Rutgers University.
Chapin served for many years as board chairman of the World Hunger Year, a project founded by his younger brother Harry.
"I only wish Jim could have lived to do the great things he had yet to do," Diana Chapin said. They were married in 1968 in Ithaca, N.Y. She served for many years as first deputy commissioner of New York City's environmental affairs department.
Besides his wife and his father, Chapin is survived by his mother, Elspeth Hart; two sons, James Derby Chapin and David Sheffield Chapin; brothers Tom and Steve; and several half-siblings.
The funeral will be private. A memorial service will be scheduled later in New York City.