U.S. District Judge Norma Holloway Johnson probably felt she had no choice but to reject President Clinton's request that Secret Service agents be given a shield that would exempt them from testifying about his relationship with former White House intern Monica Lewinsky. Nevertheless, it was a terrible decision.

Johnson noted what the Justice Department had already conceded, that the special "protective function privilege" has no basis in the Constitution or the law. In fact, that was a central argument of independent counsel Kenneth Starr, who urged the court to reject the administration's request.But the absence of a constitutional or statutory privilege does not mean the president's request was unreasonable. In fact, it means the opposite: The privilege is so reasonable, and so firmly grounded in custom and common sense, that nobody had thought to write a law to affirm it.

Thus, Johnson may have acted within the strict confines of the law when she denied the president's request, but her decision was extremely harmful, making it harder for presidents to do their important work. The ruling could even jeopardize the president's life.

Like many others who hold positions of responsibility, presidents must have private, frank conversations with their advisers. If presidents fear these candid conversations will one day be disclosed, they will be tempted to avoid the essential give-and-take that candor requires, or they may keep the Secret Service at arm's length and thus create an opportunity for a would-be assassin.

That's a choice presidents shouldn't have to make. Clinton ought to appeal Johnson's ruling, so that he and his successors will not have to make it.

In her ruling, Johnson noted that if Congress believes a protective function privilege exists, it is free to create one. Congress should interpret that as an invitation and act accordingly.

Republicans who are not disposed to give Clinton the shield he requests should reflect on the fact that, sooner or later, a Republican again will sit in the Oval Office and for any number of reasons might seek the protection Clinton requests.

Meanwhile, we hope the legal community will prevail upon Starr not to abuse the power that Johnson has given him. He is investigating the possibility that Clinton had an affair with Lewinsky and that Clinton conspired to obstruct justice by telling Lewinsky and presidential aides to lie about it.

But there are other, better ways of conducting this inquiry than by forcing the Secret Service to betray secrets.