Merlo J. Pusey, 83, a retired editorial writer at The Washington Post, a prize-winning biographer, and a notable student of the Constitution and how it should work in war and peace, died of cancer Nov. 22 at Sibley Memorial Hospital.

Mr. Pusey joined the editorial staff of The Post in 1928 and rose to be associate editor. He retired from that work in 1971, but contributed occasional pieces to the newspaper until about two years ago. In 1952, he published a two-volume biography of Charles Evans Hughes, a chief justice of the United States, for which he won the Pulitzer and Bancroft prizes.

His other books include a biography, published in 1974, of Eugene Meyer, the financier and public official who purchased The Washington Post at a bankruptcy sale in 1933. Mr. Meyer, who died in 1959, was the father of Katharine Graham, now the chairman of the board of The Washington Post Co., and a grandfather of Donald L. Graham, the present publisher of the newspaper.

Mr. Pusey's association with The Post and the Meyer and Graham families thus spanned the long period in which the enterprise went from a point close to extinction to a position of stability and health. And in his years of service this scholarly, principled, civil and tough-minded man played a major role in shaping the newspaper's editorial policy. If he sometimes failed to sway his colleagues in the give-and-take of the daily editorial meetings, he nonetheless made them reexamine their arguments. In this way he exerted a balancing and improving influence on them.

Mr. Pusey's passion was the Constitution and the rule of law. The test for him was how constitutional principles worked under the pressure of great events. One of these events -- President Franklin D. Roosevelt's ill-fated plan in 1937 to "pack" the Supreme Court -- served to sharpen the focus of Mr. Pusey's thinking. What Roosevelt proposed was to create enough new justices so that there would be a majority in favor of his New Deal economic program. The court had declared several key legislative acts unconstitional.

To Mr. Pusey, the "court packing plan" was a violation of the Constitution. In a widely circulated book called "The Supreme Court Crisis" that appeared in 1937, he said:

"In the mind of the President the chief question in the Supreme Court controversy seems to be one of economic and social reforms. To the opposition it is a question of maintaining our system of government . . . .

"Only under authoritarian rule are institutions crushed because they do not serve the immediate purpose of the group in power . . . . This does not mean, of course, that the power of the Court is superior to that of Congress. 'It only supposes,' as Hamilton pointed out a century and a half ago, 'that the power of the people is superior to both' -- and that revolutionary changes should await their approval."

More than 30 years later, Mr. Pusey published another book, "The Way We Go To War." The Constitution vests the power to declare war in Congress. Yet successive presidents had played the principal part in deploying troops in the Korean war, in the Lebanon landing of 1959, in the Dominican Republic, and most notably in Vietnam. People spoke of "the imperial presidency."

Like others who lived through World War II, Mr. Pusey did not rule out conflict as an instrument of policy. He called for a redress of power in favor of Congress. Even if presidents act in the national interest, he said, "a worthy end does not justify slick, dubious means. Democratic government is in very large measure a matter of method and procedures."

This concern with method and procedure is evident in an article Mr. Pusey published in The Post in 1982 on another controversial event: the jury verdict that found John Hinckley not guilty by reason of insanity in his assassination attempt against President Reagan. "Congress must face the necessity of relieving the judicial system from the mockery resulting from the Hinckley verdict," he wrote. "The country simply cannot tolerate a rule that makes the law an accessory to violence in utter disregard of the basic human right to life."

Last year, Mr. Pusey published a volume of poetry, "Ripples of Intuition." Its verses are about love, the changing of the seasons and the passage of years, the countryside. There are also verses about public service and politics and they speak of Mr. Pusey's continuing interest in the workings of government.

Merlo John Pusey was born on Feb. 3, 1902, on a farm at Woodruff, Utah. His parents were John Sidney and Nellie Quibell Pusey. He grew up on the farm and in Salt Lake City, where he enrolled in the Latter-day Saints University. He worked on the school newspaper and journalism became his ambition. He was a reporter and assistant city editor on the Deseret News while attending the University of Utah, where he was elected to Phi Beta Kappa and graduated in 1928.

On Sept. 5, 1928, he married the former Dorothy Richards and the couple drove east on their honeymoon. Arrived in Washington, Mr. Pusey took a temporary job with the old Washington News. Shortly thereafter, he walked into The Post and asked to see the managing editor about a job as a reporter. Instead, he saw the editor, who hired him on the spot as an editorial writer.

From 1931 to 1933, in addition to his newspaper work, Mr. Pusey was a part-time member of the staff of the Senate Finance Committee. From 1939 to 1942, he was an instructor in journalism at George Washington University. His interest in Roosevelt's "court packing plan" led naturally to his biography of Hughes, who was chief justice at the time and who gave him a number of interviews and full access to his private papers. Other books include "Big Government: Can We Control It?" (1945) "Eisenhower the President" (1956), and "The USA Astride the World" (1971).

Mr. Pusey, who lived on a farm in Dickerson, Md., was a member of the American Political Science Association and the Cosmos and National Press clubs. He was a member of the Chevy Chase Ward of the Church of Jesus Christ of Latter-day Saints.

In addition to his wife, of Dickerson, survivors include three sons, C. Richards and David R. Pusey, both also of Dickerson, and John R. Pusey of Orem, Utah; two brothers, P. Fred Pusey of Ogden, Utah, and Glen Pusey of New York; ten grandchildren, and seven great-grandchildren.