Since a divided Canada would be a weaker friend and neighbor, America has a big stake in the outcome of the renewed threat by Quebec to leave the 123-year-old confederation.

The threat of political separation has become so serious that Prime Minister Brian Mulroney flew to Toronto and to Quebec City last week to make two formal appeals in English and French in an effort to hold Canada together.Mulroney, however, had no new proposals and told Canadians nothing they didn't already know about the unhappy consequences of separation.

The trouble is that some other provinces are too inflexible and, in response, Quebec has become too demanding. The rift could have been healed last June if all 10 provinces had accepted a new agreement recognizing Quebec as a "distinct society" because of its French heritage. But three provinces balked at an accord that had to be approved unanimously.

In response, Quebec is demanding that Ottawa hand over power in major fields from energy and the environment to justice. If not, Quebec is to hold a referendum next year on sovereignty.

French-speaking Quebec is Canada's largest province in area and second largest in population. A breakup of any kind would leave English-speaking Canada broken into two geographically isolated pieces. Though Quebec has increased its foreign trade since 1980, its economy is still an integral part of the Canadian economy. Moreover, Quebec still sells far more products to the rest of Canada than it does to the United States.

The continued political uncertainty is no inducement to foreign investment. Consequently, as long as the threat of political schizophrenia persists, all of Canada will suffer. The threat, in turn, will end only if Canada learns that cultural and linguist differences need not preclude unity and harmony in a free confederation.