WITH extraordinarily bad timing, special prosecutor Whitney North Seymour Jr. has precipitated an international flap that has ramifications far beyond Embassy Row. In furtherance of his case against former White House aide Michael Deaver, Mr. Seymour has attempted to subpoena the Canadian ambassador and his wife and force both to testify in court. It is so clear, on the basis of longstanding international practice and the specific terms of the Vienna Convention, that ambassadors are protected by diplomatic immunity from such process that legal experts have expressed astonishment at the tactic. The State Department -- mindful of the status of American diplomats abroad -- went to court immediately on behalf of the Canadians.

This dispute is particularly unwelcome now because it adds another complication to the debate on the whole concept of special prosecutors, also known as independent counsels. Mr. Seymour has been appointed by a special panel of three federal judges to look into charges involving Mr. Deaver. Under the provisions of a 1978 law, he is to conduct the investigation and proceed with prosecution independently, and the Justice Department has no control over what he does or how he does it. The constitutionality of that law is now being contested in court by Mr. Deaver and, in a separate action, by Lt. Col. Oliver North. In addition, the law itself will expire early next year, and Congress is in the process of deciding whether to extend it, amend it or make it permanent.

At the center of the debate in court and Congress is the idea of transferring tremendous prosecutorial power to an individual who is responsible to no one else in government. Is this constitutional? Is it wise? Is it fair? Are there alternatives for dealing with alleged wrongdoers in high government office? What should be done about an independent counsel who abuses his powers? Should the ethics and conflict-of-interest regulations that apply to all government attorneys be imposed on special prosecutors? Does the attorney general have enough discretion -- or too much -- in deciding whether to initiate an appointment? These are important questions that have been festering for years but are no longer academic because investigations are now leading to prosecutions.

Now, in spite of overwhelming legal opinion and State Department advice to the contrary, Mr. Seymour has attempted to do what no other federal prosecutor would do by proceeding against an ambassador who has diplomatic immunity. In taking this course he has provoked a whole new set of questions about the powers and responsibilities of his office. He would be wise to concede that the subpoena was a bad idea and let judges and legislators consider the law without worrying about its peripheral effect on treaties and international obligations.