ONE BIT OF UNFINISHED congressional business is rewriting the law governing public disclosure by lobbyists. The public now knows far too little about the efforts and finances of thousands of organizations that jockey for influence on Capitol Hill. But how much should a lobbying group have to disclose? Reports should certainly include some basic facts - the bills or issue that each group is interested in, the names of employees or agents who lobby, how much they are paid, and other expenditures, including gifts to legislators and their staffs. Beyond that, though, the principles vital to a healty democracy.

We are not primarily concerned about the possibility that detailed reporting would be too burdensome for smaller lobbying groups. What troubles us more is the potential for official instrusion as largely, if not wholly, beyond the reach of government. One example would ne requiring membership organizations to disclose the names of individual contributors.

Then there is the matter of indirect or grass-roots lobbying, the popular and often effective means of putting pressure on lawmakers by generating letters or calls from constituents and other citizens. Of course it would be interesting to know who is behind such organized, skillful campaigns. But should a veterans' lobby have to file a report on every newsletter that urges its members to write to Congress about a bill? Should a corporation with a paid lobbyist have to report and interoffice memo that asks its employees to sign petitions for tax changes or import controls? A bill sponsored by Sen. Edward M. Kennedy (D-Mass.) and others would require such disclosures.

A measure before the House Judiciary Committee would exempt regular publications, such as union newspapers or corporate quartely reports, that incidentally advocate contacts with Congress about bills. But the House bill would still require lobbying groups to file a full description or a copy of every ad or special bulletin encouraging citizen's appeals to the Senate or House.

This is disclosure run amok. The goernment would be collecting mountains of material from thousands of private groups across the whole spectrum of civic, economic, social and political activity. These groups would have to report more about their efforts to petitons Congress than political candidates have to disclosure about the substance of their ads or mailings with the government. For that matter, senators and representatives do not have to report on their own efforts to drum up support for various programs and policies. And proposals along those lines would rightly raise a mighty fuss about interference with First Amendment rights. Yet mandatory disclosure of groups' communications involves precisely the same dangers. Free discussion of public issues could be inhibited.

Thus the risks involved in this kind of disclosure seem to us to outweigh any gains in terms of public understanding of the workings of governemnt. Organizations' appeals to supporters, after all, are just one aspect of the complex process of shaping public opinion and policies. What prompts a citizen write a senator may matter less than how fervent that citizen's opinion is and how much it will influence his vote in the next election. No lobby-reporting law can throw much light on that. All in all, we think Rep. Don Edwards (D-Calif.), Sens. Charles McC. Mathias Jr. (R-Md.) and Edmund S. Muskie (D-maine) and the American Civil Liberties Union are on a better track. They advocate disclosure of the basic facts about lobbyinsts' Capitol Hill activities - but would leave grassroots communications alone.