"CASTING MORE LIGHT on lobbying" sounds like a fine good-government goal. But it's the kind of formulation that members of Congress should be wary of as they try once more to rewrite the lobby-disclosure law. This is one of the fields in which opeinness -- meaning broad, detailed, compulsory disclosure -- is not at all synonymous with free, healthy political activity. Congress cannot push lobby-reporting laws very far without intruding on realms of private association and expression that are beyond the proper reach of government.

A good example is the burgeoning field of so-called grass-roots or indirect lobbying -- atempts to influence senators and representatives by stirring up mail, phone calls and visits from people back home. Such campaigns have become so commonplace that lawmakers are constantly being bombarded from many sides. They find it quite discomfiting. They are not always sure how much real political force a barrage of postcards represents. And the recent surge in sophisticated grass-roots campaigns by business and single-issue groups has fed suspicions that lobbies with large bankrolls and extensive mailing lists are gaining undue influence on Capitol Hill.

Should groups that organize indirect lobbying have to report on their financing and activities? That would certainly help citizens and officials find out who is behind these campaigns. But consider what such a sweeping law would mean. Every active group with legislative concerns -- including trade associations, unions, universities, charitable societies and citizens' groups -- would have to report to a federal agency on its meetings mailings, advertisements and other issue-oriented activities. Anyone suspected of non-compliance would be subject to federal audits, investigations and penalties.

Talk about overregulation! The paperwork would be incredible. Much more ominous is the whole idea that private groups should be compelled to report on perfectly legitimate communications with their own members, supporters and the public at large. The chief advocates of full disclosure say they don't want to interfere with any group. Last year's House debate suggested, however, that some congressmen do see disclosure as a way of embarrassing or burdening interest groups whose lobbying they find bothersome.

So far, enough lawmakers have recognized these and other problems so that no overreaching bill has gotten through. A House judiciary subcommittee is now tackling the subject again. The White House has been seeking compromises, but a coalition of interest groups -- ranging across the spectrum from business associations to the Sierra Club -- is insisting on a carefully limited bill. Their position may sound self-serving, but it really serves the national interest in free discussion of public affairs.