Rene Levesque, the leader of Quebec's Separatists, who want to take this predominantly French-speaking province out of Canada, resorted to imagery the other night as he was drawing the line for the coming battle.

"It is like one of those days in August," he said, "when one-half of the sky is very bright and the other half is dark with clouds letting little light through."

For the separatists, of course, theirs was the bright side. For those who favor continued Quebec participation in the Canadian federation, Levesque's plan is a throwback to the dark ages of tribalism.

But for the majority of French Canadians, who comprise almost one-third of Canada's 23 million people, the battle line is blurred. Their attachment to their heritage is strong and the idea of sovereignty seems attractive. Yet they have doubts about the prospective great adventure that would break up Canada and leave them alone in perilous international waters at a time of great economic uncertainty.

The electoral arithmetic in Quebec, poll after poll, shows that three-fifths of its 6.2 million people have made up their minds. Forty percent, all of them French-speakers, are split between dedicated separatists and unconditional federalists. In addition, the 20 percent of Quebec's population that is English-speaking is opposed to separation.

This leaves the remaining 40 percent as the focus of the separatist Parti Quebecois campaign in the forth-coming referendum on sovereignty.

The outcome of recent federal elections has injected a new element in the situation here. On its face the elections produced a breakdown in the national political party system between the English Canada of the conservatives and the French Canada of the Liberals.

In a country with as weak a sense of national identity as Canada, this is a serious matter, especially since French Canada is almost unrepresented in the new national government.

The election outcome also bolstered the two-nation theory of Canada that is held by most people in Quebec. The alternative to the two-nation concept, represented here by former prime minister Pierre Trudeau and his vision of a unified bilingual nation, was defeated in the May 22 elections.

But there are two distinct views among Quebec people on how to proceed from here.

Levesque argues that the new political configuration in Ottawa merely reinforces his belief that Quebec must become a sovereign state with all the essential powers of a nation.

Claude Ryan, the Quebec Liberal leader, believes that Quebec's position within Canada must be reinforced so as to have all the powers of a nation without seeking political independence. Ryan has long argued that the acceptance of two nations in Canada should be the starting point for new constitutional negotiations.

In short, Quebec will be faced with a clear choice in the coming referendum debate.

An editorial in LeDevoir, the influential Montreal daily assessed the situation this way:

"The coming to power of Joe Clark and the new composition of the Parliament create for Quebec an entirely different context which can turn out to be healthier and clearer. Now the center of gravity of the Canadian crisis will be displaced toward Quebec which will thus recapture the debate on its political future."

Clark, who has thus far failed to attract major Quebec figures into his Cabinet, is likely to avoid involvement in the constitutional future of Canada before the sovereignty referendum and to let Quebec carry out its own debate.

On Sunday, Levesque won an over whelming endorsement from his party to seek sovereignty but to keep Quebec in an economic association with the rest of Canada. English Canada, he believes, would eventually accept such an economic arrangement involving a customs and monetary union because of practical and geographic considerations.

Ryan and his supporters are arguing that the possibility of painless sovereignty is an illusion and that Quebec would be better off by gaining "cultural sovereignty" and greater political autonomy within a looser Canadian federation.

One of the great difficulties for the Separatists is that they have come to power there at a time of economic crises in the West. Economic uncertainties coupled with high unemployment may tend to make the proverbially cautious Quebec people more reluctant to embark on dramatic ventures regardless of how attractive they may appear.

A sovereign Quebec outside Canada's financial and economic frame work would have to demand enormous sacrifices from its citizens. Furthermore, with the eclipse of central Canada by the newly rich western provinces it is unlikely that English speaking Canadians would extend economic benefits to Quebec if it breaks up the country.

Ryan and his supporters will emphasize the prospect of an uncertain future. Levesque's major weapon will be an appeal to Quebec nationalism - "that we be masters in our own house." CAPTION: Picture, Katherine Clark, 2, applauds her father, Joe Clark, at a party celebrating the 40th birthday of Canada's new prime minister. Her mother is holding her. United Press Canada