Justice Harry A. Blackmun has done what both Presidential candidates fear to do: make the Supreme Court an election issue. While denying "taking sides in this business," he has nonetheless done just that. His statements, in two speeches, are extraordinary - and could prove costly for the court and the country.

The 79-year-old jurist is clearly upset by the "shift in emphasis" brought about by President Reagan's four conservative appointees. And he warns of more dramatic changes after the election - changes posed by the prospect of his own departure and that of some of his colleagues.If the court becomes an election issue, no subject could be more controversial than the 1973 ruling that women have a right to obtain abortions, which Justice Blackmun authored. And that is the very issue the Justice repeatedly emphasizes: "Will Roe v. Wade be overturned?"

Justice Blackmun is sounding political sirens. Roe, he stresses, symbolizes "the progress of the emancipation of women." In a speech in Little Rock, he was blunt: "There's a very distinct possibility that (Roe) will (be reversed) - this term." This prospect is underscored by the fact that two abortion cases, which could provide the opportunity for a 5 to 4 reversal, are rapidly nearing the docket.

Such off-the-bench remarks, coming from a sitting Justice, are cause for concern. This is so even though in the last decade several Justices have abandoned the traditional code of "judicial lockjaw," as Justice Felix Frankfurter called it.

Greater openness about how the court works may improve public understanding, but the danger remains that the court's institutional power and prestige will suffer when justices plead their cases to the public.

Justice Blackmun's alarmist predictions are disturbing for another reason: They may be misleading. One cannot with any certainty count five justices who are presently prepared to level the Roe edifice. It is impossible to predict who the next president will name to the court and how his appointees will vote in the long run.

The reason for Blackmun going public may be twofold. First, perhaps he wants the presidential candidates to talk openly about the court and federal judiciary. Would Gov. Michael S. Dukakis or Vice President Bush follow President Reagan's example and pack the courts with their ideological clones?

Secondly, Blackmun may be sending a subtle message to the Democratic-controlled Senate: Beware a sharp rightward shift with the confirmation of the next nominee. Roe would be in real danger. Do not confirm anyone who would overturn it.

Undoubtedly, Blackmun deems the judicial stakes in the election to be high enough to justify his going public.

Apparently, he either rejects or does not believe that the court's status depends on the public perception that justices stay above the political fray.

The court, to be sure, is an election issue despite the fact that neither Bush nor Dukakis wants to make much of it. But the candidates, not the justices, ought to be the players in the public debate over the court.

Justice Blackmun's fault lies not only in what he says, but in his saying it.