QUEBEC FACES A DECISION NEXT Tuesday whose implications may be as ominous as was South Carolina's decision in 1860 to withdraw from the United States.

No matter which way the vote goes on whether to negotiate political sovereignty and economic association with the rest of Canada, the Quebec referendum will probably produce a tense situation.

Quebec militants are not likely to accept an unfavorable outcome, regardless of how close the vote. They would no longer be satisfied with Parti Quebecois leader Rene Levesque's moderate, constitutional approach, demanding instead that he declare Quebec independent or give way for a new leader.

But Prime Minister Pierre Trudeau warned two years ago that he would use force to counter any illegal Quebec action to achieve independence. Trudeau, it will be remembered, invoked the War Measures Act in October 1970 and sent federal troops to Montreal to restore order after provincial authorities were unable to cope with spreading violence there. If the referendum does fail, then, there is a serious possibility that violence will break out in Montreal and perhaps other cities, and that Trudeau would again send in troops. In that case, the specter of civil war would hang over Canada.

In this crucial situation, much would depend on Levesque's intentions, how strongly he might condemn such disorders and how firm he would be with subordinates who waver in the face of violence.

On the other hand, if the vote is affirmative, Levesque is unlikely to obtain Canada's concurrence with his plans. Thus far, Ottawa has made clear that it will not negotiate sovereignty as well as economic association. In that event, Quebecois militants would still pressure Levesque to declare independence, even though Levesque himself probably would want to seek a new mandate before moving toward any complete break.

Either way, the implications for U.S. policy toward Canada are large. There may no longer be one peaceful border to the north, speaking with a unified voice in world affairs. The huge trade and investment the United States enjoys with Canada could suffer irreparable damage. Given Quebec's strategic location, moreover, its possible secession from Canada could bring serious problems for North American defense and for international shipping on the St. Lawrence Seaway.

Although Levesque has assured both Canadians and Americans that an independent Quebec would join NATO and NORAD, its participation is less certain if Quebec's independence results from acrimonious negotiations or violence by some political faction.

U.S. policy since 1976 has been to treat the Quebec issue as an internal Canadian affair. That is no longer adequate. The question is whether the United States can be helpful in resolving this dangerous situation before it becomes critical.

The most urgent change Washington needs to make is to show understanding for the Quebecers' desire for significant changes in their status within Canada. Such a posture would be supported by 80 percent of Quebecers and should be acceptable also to Ottawa. Both Pierre Trudeau and federal opposition leader Joe Clark are committed to constitutiuonal reform from which all provinces would benefit, and many Quebecers who now support Levesque might accept a new form of federalism, particularly if it grants Quebec a large degree of autonomy.

The United States should offer to play honest broker if and when negotiations begin between Quebec and Ottawa. Washington need not take the lead in suggesting solutions, but it ought to be willing to serve as a consultant to the parties if they ask.

Washington also should adopt a forthright public stance regarding defense of North America in case Quebec becomes independent. The United States should state clearly that it considers the security of Canada to be not only a vital U.S. national interest but one that approaches the survival level. The importance of Canada to the priorities of the United States cannot be exaggerated: Canada is the heart of the U.S. continental defense and essential to America's economic well-being. No one should doubt that America will not tolerate an unfriendly state on its northern border, particularly if it seeks links with Communist states.

Nevertheless, the United States should also be prepared to accept the idea of self-determination for Quebec if independence is accomplished through constitutional means and if it is reasonable clear that the long U.S. friendship with Quebec will continue. We should realize, as many English-speaking Canadians have concluded, that a friendly, cooperative and independent Quebec is preferable to an unhappy, uncooperative and insecure province within confederation.