THE COMMANDING ISSUE in Canada's election next Tuesday will be the fundamental one-whether, and how, to continue as one nation. No sudden dramatic changes are likely to follow the election immediately, since both of the major parties are committed to the principle of national unity. But the vote will be a response to a sense that national dissolution has become a genuine possibility.

Separatism has accelerated in the last decade, which is to say during the tenure of Prime Minister Pierre Elliott Trudeau and his Liberal Party. He proposes to continue to meet it with a subtle and flexible policy that amounts to playing for time and negotiating the best deal that the contentious provinces will accept. The Progressive Conservatives under Joe Clark intend to set sharper limits on the negotiations-although it's not entirely clear how they would enforce those limits.

What might Canada look like after separation? If Quebec were to depart, drawn by its different language and cultural traditions, the western provinces might follow that example. Opponents of separatism say that the result would be, in effect, Central America with snow. They suggest a checkerboard of accidental republics, none too stable or prosperous, all dominated by the economy of a neighbor that is huge, wealthy and heedless. The supporters of separatism argue that, on the contrary, the model is Scandinavia.

On the face of it, the Scandinavian possibility is more plausible. But the analogy is far from exact. Each of the Scandinavian nations has its own language, setting its people apart. More important, the events of this century-particularly World War II-created a vast psychological distance between them and Europe's economic powerhouse, Germany. On this continent, with its shorter and happier histories, political communities are not so clearly cut. New countries in the north would find it difficult to establish national identities. The precedent of ethnic secession, once established, might go further than its authors intend.

But that's an extreme view of the Canadian future. The likeliest short-term outcome is still a good deal short of secession even by Quebec. It is rather a loosening of the federal hold on provinces' domestic policies-an evolution of something that Quebec can call cultural sovereignty and that Ottawa can call enlightened federalism. Most Canadians seem to accept that prospect, at least in principle. But to accomplish it will require enormous political skill and vision. The election is an uneasy choice between the elegantly intellectual Mr. Trudeau, who after 11 years has perhaps been in office too long, and the stolid, dogged Mr. Clark, who has never been in high office at all.