DRAWING A CAREFUL distinction between legislative duties and political undertakings, the Supreme Court ruled last Tuesday that members of Congress can be sued for libelous material contained in their press releases and newsletters. It is a distinction that flows directly from earlier court rulings on the scope of congressional immunity, but it is one that, nevertheless, presents troubling problems.

At first glance, the court's ruling makes sense. Why should the writings of a congressman have more protection against libel suits than those of a corporation or a news paper or an ordinary citizen? Ruputations can be needlessly ruined by press released as well as by news stories. Issuing press releases, after all, is not the principal job of a member of Congress.

That was the line of analysis the court followed. Anything a member says on the floor of Congress is privileged, as is anything that occurs in its committee rooms or in internal communications among members. But once a congressman begins to communicate directly with the public, the shield of immunity for matters dealing with legislation or deliberations disappears.

This logic may be flawless; eight of the nine justices seem to think it is. But rest on a view of the job of a congressman that is peculiarly narrow. Perhaps legislation was the product mainly of debate and deliberation conducted inside the halls of Congress in the days when communications with constituents were limited. But today it is often the product of heavy two-way communication between the members and those they represent. It is hard in the real world -- though perhaps not in legal theory -- to separate the job of a congressman into two tidy packages, one marked communicating with members and the other marked communicating with the rest of the world.

The immediate effect of the decision is that Sen. Williams Proxmire [D - Wis.] will have to defend himself against a charge that he libeled a research professor in a press release announcing one of his "Golden Fleece" awards. Other members will learn from his experience -- whether he wins or loses -- to be quite careful about the blasts they issue at alleged bureaucratic or citizen incompetence and corruption. The legal costs of defending against a libel suit -- even a frivolous one -- can be high.

In that sense, the court's decision could have one useful side effect. The costs of litigation are out of control, and the courts are doing little about it. Confronted with having to pay such bills themselves, members of Congress may start thinking of ways to reduce them.