How influential is Brookings? As a research organization, Brookings does not try to design legislation. ''It's awfully hard to find a law that somebody in a think tank proposed and that got legislated,'' says Alice Rivlin, director of economic studies. Brookings researchers, however, played a significant role in establishing the Congressional Budget Office in 1975 - whose first director was Ms. Rivlin. Easier to measure, perhaps, is the effect on policymakers of warning signals sent up by researchers. ''I think my two great services to the Democratic Party have been negative,'' says Brookings fellow Charles L. Schultze,'' who was director of the US Bureau of the Budget under President Johnson and later chairman of President Carter's Council of Economic Advisers. He refers to his writings, as a Brookings scholar, and his testimony at congressional hearings, which helped, he says, to ''defang'' the controversial Humphrey-Hawkins bill, which sought to guarantee full employment. Strongly supported by the Democrats, it became law in 1978 in a ''watered down'' version. And last fall he distanced himself again from more liberal thinkers by writing an article for the Brookings Review titled ''Industrial Policy: A Dissent.'' Directed against a policy of government intervention to prop up failing industries, the article is credited with helping to keep a major ''industrial policy'' plank out of in this year's Democratic platform.