THE PERJURY trial of former White House aide Michael Deaver is set to begin today. An independent counsel, Whitney North Seymour Jr., is the prosecutor. Mr. Seymour's initial investigation centered around Mr. Deaver's alleged lobbying activity both before and after he left the White House in 1985 and whether this activity was in violation of any federal conflict-of-interest laws. The indictment, however, is not for substantive ethical violations but on the weaker charge of perjury. Specifically -- and this is particularly hard to prove -- Mr. Deaver is charged with lying under oath when he testified that he did not remember having discussed the question of acid rain at a lunch with Canadian Ambassador Allan Gotlieb and his wife on a certain date.

To win a conviction, Mr. North apparently believes he must have the testimony of the ambassador concerning the luncheon meeting. No one other than the Gotliebs and the Deavers was present. But the Canadians have invoked diplomatic immunity, which protects them from being compelled to testify in court. Last May, when the prosecutor attempted to subpoena Ambassador and Mrs. Gotlieb, a first-class flap ensued, and diplomatic and legal experts, citing longstanding international practice and the specific terms of the Vienna Convention, were unanimous in protesting the action. Judge Thomas Penfield Jackson quashed the subpoenas, and the matter was thought to be settled.

Now though, on the very eve of the trial, Mr. Seymour has made a last-ditch attempt to get the Gotliebs to testify. In a letter to an attorney who represents the Canadian government, Mr. Seymour issued a not-very-veiled threat: unless the ambassador testifies, Mr. Seymour wrote, he will be "forced to place much greater emphasis at trial on the unlawful acts engaged in by Deaver when he was working for the Canadian government." Never mind that the defendant has not been charged with any such unlawful acts, the letter was, as the Canadians interpreted it, an attempt to intimidate and embarrass that government.

The State Department demands that Mr. Seymour live up to his agreement to communicate with foreign governments only through proper channels. The Justice Department, which has no power to regulate the activities of an independent counsel, has asked the court to order Mr. Seymour to direct all future communications with the Canadians through the State Department. Officials in both departments are right; more is at stake in our relationship with Canada than the prosecution of this case. Mr. Seymour's frustration at not being able to subpoena the ambassador to boost his case is evident. But he is way out of bounds in pursuing this harassment, and by his conduct he jeopardizes the future of the independent counsel as a permanent institutio