The final, decisive act in Canada's national election is being played out here in Ontario.

"The Province of Opportunity," as Ontario styles itself, has become the target of national political leaders in this last week of the campaign.

According to a Canadian saying, Ontario has a third of the population and half of everything else. Its voters will elect 95 - or over a third - of the new Parliament's 282 members.

Moreover, Ontario is the stage for the final and toughest campaign battles since the rest of the country, by and large, has made up its mind about Prime Minster Peirre Trudeau and his Liberal Party and the Conservatives and their leader Joe Clark.

The Liberals are expected to win between 60 and 65 of the total 75 seats in Quebec largely because its French Canadian majority votes along tribal lines. As one Quebec resident said recently about Trudeau, "He is a son of a bitch but he is our son of bitch."

West of Ontario are four provinces of English Canada Manitoba, Saskatchewan, Alberta and British Columbia) with 77 seats. The Conservatives are expected to get 60 to 65 of those. There is no "native son" affection for the Albertan Clark there in the way there is for Trudeau in Quebec. But Trudeau is so disliked in the West that references to him move rapidly to four-letter words.

The four Maritime provinces (New Brunswick, Nova Scotia, Newfoundland and Prince Edward Island) have a total of 32 seats and Conservatives are expected to win more than half of those.

The eletoral arithmetic before Tuesday's vote thus narrows down to Ontario and its 95 votes.

Ontarians see themselves as the voice of common sense and reason, pursuing a vision of a modern large Canadian federation among parochial provinces preoccupied with regional issues and provincial boundaries.

Trudeau's 11 years' tenure as prime minister was grounded in Quebec and Ontario. In 1968 he won the hearts of the rising urban middle class heavily concentrated in Ontario and these voters have relected him in 1972 and 1974.

But while Ontario remains the crucial province because a third of Canada's 23 million people live there, its economic preeminence and power are no longer unchallenged.

In this decade the economic center of gravity has been shifting toward the prosperous West, which is the fastest growing and richest part of Canada. That English-speaking region traditionally leans toward the Conservatives. But during Trudeau's years the Liberals have collapsed as a serious party in the West and today do not have a single deputy in any provincial legislature west of Ontario.

The Conservatives, who controlled Ontario's provincial government for 36 consecutive years, have built their electoral strategy on winning the West and Ontario. They must win 50 seats in Ontario to form a minority government and 60 to form a majority government.

The latest Gallup poll shows the Conservatives almost 10 points ahead in the province. Clark's proposal to make parts of property taxes and home mortgage interest tax deductible, accounts to a large extent for the strong Conservative showing. The Liberals have found themselves on the defensive by opposing the mortgage plan, although they had enacted a similar measure limited to businesses and corporations in 1978.

The mood among Ontarians is for a change. They have been hit by 9 percent inflation, mortgage rates of 11 percent and a declining dollar. They are also irritated by what they see as Trudeau's arrogance.

Clark has sought to capitalize on their dislike of Trudeau by attacking the prime minister's record and by avoiding serious gaffes that could reinforce already strong public doubts about the 39-year-old Conservative leader's ability to run the country. But political observers say signs indicate that in the past 10 days Clark's vagueness on substantive issues may be backfiring here in Ontario.

Another element in the bitter election struggle here is the sudden surge in popularity of Edward Broadbent, leader of the socialist New Demoncratic Party. It is not clear, however, whether the fact that he made a strong showing during a nationally televised debate with the other two candidates will translate into votes for the socialists.

Canadian commentators generally predict a clark victory. Yet despite the polls the race here is extremely close. While the threat of Quebec leaving the Canadian federation has not figured prominently in the public debates, it still weighs heavily on people's minds.

Should Ontario join the rest of English Canada to elect Clark, this Would in effect mean a polarization of the country between English Canada and French Canada, with the latter having virtually no representation in Ottawa.

It is argued here that the Ontarians would not welcome such a development and that while they may hate Trudeau they could rationalize giving their votes to him as the man from Quebec who can keep the country together. CAPTION: Picture, Joe Clark, leader of Canada's Conservatives, joins local party candidates at a rally in Montreal. UPI