Richard E. Neustadt, 84, a leading presidential scholar of the last four decades and longtime Harvard University professor who stepped off the campus to advise presidents, died Oct. 31 at his home in Furneux Pelham, a village in Hertfordshire, England, of complications from a fall about a week ago.

Although he had a home on Cape Cod, his main residence was in England, where his wife, Shirley Williams, is leader of the Liberal Democrats in the House of Lords.

Dr. Neustadt, mentor to presidents and presidential candidates, was perhaps best known for his books "Presidential Power" (1960) and "Preparing to Be President" (2000).

Many reviewers noted that his proximity to presidential power distinguished his books from others that poured out of academia. Helen Fuller, writing in the New Republic in 1961, noted the "unusual ring of authenticity" in Dr. Neustadt's writing.

He served in President Harry S. Truman's White House, counseled Democratic presidents from John F. Kennedy to Bill Clinton and was former vice president Al Gore's thesis adviser at Harvard. He was the 1972 chairman of the Democratic National Convention's platform committee.

"Presidential Power," his seminal work, was updated several times. It examined how a president turns his authority into the actual exercise of leadership. The real power of the presidency, he said, was "the power to persuade."

Dr. Neustadt, a professor emeritus at Harvard's Kennedy School of Government and founding director of its Institute of Politics, was a recipient last year of the Paul Peck Presidential Award honoring distinguished service to the presidency. The award is funded by a U.S. Customs Service employee who donated millions of dollars to the National Portrait Gallery to enhance its presidential programs.

Gore, in an interview, said of Dr. Neustadt: "He had a rare turn of mind with an unusual ability to blend brutal common sense with highly specialized analysis. I took his course when I was an English major, and it was the principal reason I changed my course of study."

Clinton said in a statement: "His many books on the American presidency remain some of the most influential and enlightening in history. Professor Neustadt spent a lifetime advancing the public understanding of the American presidency. I am grateful for the friendship and wise counsel he gave to me."

Richard Elliott Neustadt was born in Philadelphia and raised in San Francisco and then Washington, where his father, a Social Security board official, was a policy and personnel adviser to Presidents Woodrow Wilson and Franklin D. Roosevelt.

He was a graduate of the University of California at Berkeley and received master's and doctoral degrees from Harvard. He was an assistant economist in the Office of Price Administration, then entered the Navy in 1942, serving on the Aleutian Islands as a supply officer.

After the war, he returned to Washington and became assistant to the director of the Bureau of the Budget.

In 1951, he joined Truman's White House staff as a policy and administrative adviser. Although living in Washington during the height of the New Deal triggered an early interest in government, he once said he "really became infected with politics in Mr. Truman's school."

He joined the Columbia University faculty in 1954 and moved to Harvard in 1965.

During the 1960s, he was on the Democrats' platform committees and was a consultant to the Bureau of the Budget, the State Department, the Defense Department, the Atomic Energy Commission, Rand Corp. and the Atomic Energy Commission.

In 1977, he was a consultant to the president's reorganization project in the Office of Management and Budget.

He wrote a transition memo in 1980 for James A. Baker III, President Ronald Reagan's incoming chief of staff, and advised Clinton on his transition into the White House in 1992.

Among his other books were "Alliance Politics" (1970); "Thinking in Time" (1986), co-written with former Harvard dean Ernest R. May; and "Report to JFK: The Skybolt Crisis in Perspective" (1999).

The last book, building off his original research for "Alliance Politics," examined the cancellation of the Skybolt missile program in the early 1960s and the ensuing fallout on Anglo-American relations.

He received several top awards from the American Political Science Association.

One of his little-known private interests, turtle counting, required the same sort of quiet diligence and patience that distinguished his best academic research.

His home on Cape Cod bordered a pond, and he liked to slip into a canoe, paddle over to sunbathing turtles and count them silently before they sensed the presence of humans and left.

His wife of 39 years, Bertha Cummings "Bert" Neustadt, an educator, died in 1984. A son from his first marriage, Richard M. Neustadt, died in a rafting accident on California's Yuba River in 1995.

Survivors include his wife, whom he married in 1987; a daughter from his first marriage, Elizabeth Neustadt of London; a stepdaughter; and three grandchildren.

Richard E. Neustadt served in President Harry S. Truman's White House and counseled several Democratic presidents.