WHEN WALTER W. HELLER became President Kennedy's chief economic adviser in 1961, the new Keynesian ideas were percolating widely through Washington. It was a moment when a man of vigorous intellect could turn them into a powerful instrument for expanding and managing the country's prosperity. Mr. Heller seized his opportunity, and the next four years were a time of extraordinarily successful economic policy -- the most successful, for Americans, of this century. Those are the years that, 20 years later, the Reagan economists claimed as the model for their own supply-side strategy.

The essence of Keynesian theory is its ability to lift an economy's growth rate and push it toward full employment. After three recessions in eight years under the Eisenhower administration, there was plenty of slack in the economy and it took off, in the early 1960s, in a surge of growth unprecedented in peacetime. The danger, of course, was inflation. Mr. Heller fought it with the wage guidelines that he developed under the second president he served, Lyndon Johnson. That led to bruising battles with the steel and automobile industries, but the president, with Mr. Heller urging him on, won far more than he lost. In the years that Mr. Heller was chairman of the Council of Economic Advisers, from 1961 to late 1964, the consumer price index never rose as high as 2 percent a year. The trouble started after he left, as the first effects of the Vietnam War began to reach prices.

While Mr. Heller was a vigorous partisan, he was also an honest one who accepted both sides of the Keynesian bargain. He favored tax cuts when the economy was under capacity, and there was a big cut in early 1964. But he also supported tax increases when signs of strain appeared. Even after leaving office he continued to write memos to the president, pressing for tax increases to finance the war. For reasons that had little to do with economics, President Johnson delayed until too late, initiating the great wave of inflation that ran for the next two decades.

Mr. Heller was a man of great warmth and great sympathy for liberal Democrats' overriding concern with employment and the condition of working people. That earned him an unusual degree of trust among his party's politicians and even among labor leaders, who resisted and denounced his wage guidelines. Through his long subsequent career at the University of Minnesota he retained a sharp interest in policy and politics, returning here frequently to advise and to testify, until his death, at the age of 71, on Monday.