WITH MUCH THE GRACE and modesty that have marked his public career since he came to Washington in 1965 as Secretary of Health, Education and Welfare, John Gardner is slipping off quietly today as chairman of Common Cause. He is passing on the leadership of the public-affairs lobbying group that he founded in the spring of 1970 and that now has 250,000 members. Mr. Gardner's purpose in creating Common Cause was to be not a power broker but a power examiner: Is the government - and those entrusted to run it - accessible, accountable, responsive? In seeking answers to that fundamental question, Common Cause has won its share of victories in both the court of law and the court of public opinion.

Causes are as common in Washington as platforms from which to promote them, but we think Mr. Gardner's organization has grown to strength because it has been mindful of details, not postures.Being outraged without knowing how the political system works counts for little. Matching the two, Common Cause worked for four years to end the congressional seniority system. Its lawsuit forced disclosure of the hidden financing of the 1972 Nixon campaign. It established the principle of open government in most congressional committees and in a majority of state legislatures. In the House, it successfully took on such supposedly immovable objects as Wayne Hays, Edward Hebert and Robert Sikes. On the other side of the ledger, Common Cause has taken its share of defeats, although some of the lost causes have hung on to another day to be picked up by others. Many of the Carter administration's ideas to increase auto efficiency or to conserve energy were part of the organization's 1975 efforts to pass energy legislation.

We have had our share of differences with Common Cause and have used this space to express those differences. Indeed, around Washington it is hard to find any group - from the National Association of Manufactureres to the AFL-CIO to the Ralph Nader satellites - that at one time or another has not been put off by a Common Cause idea. But because the strength of the organization has been a combination of Mr. Gardner's own integrity and the collective vigor of the membership, its voice is listened to and its counsel sought. Its appeal has been, as Mr. Gardner has noted, in the demonstration "that citizens need not be intimidated by the vastness, the impersonality and the technical character of modern life, nor by the huge resources in the hands of those who wield power." As for the wins and losses, Mr. Gardner believes that "in any society, it is inevitable that equally worthy groups want mutually incompatible things. Unless we want such differences settled by the whims of a dictator, or unless we want to shoot it out, we must turn to the much-maligned areas of politics."

In voluntarily yielding leadership of Common Cause, John Gardner has resisted the temptation, common to Washington's movers and shakers, to hang on to the very end, be it happy or bitter. By deciding that it is time to step down, Mr. Gardner puts himself in the company of those pursuers of excellence who are more interested in advancing the process of reform than in seizing and maintaining control of it. That has been the strength of Common Cause all along: citizens joining what John Gardner called the "endless losing and regaining of balance."