Canada, like the United States, is a half-continent-size country which has been struggling, almost from its birth, to hold together as a nation.

In the United States, the North and South fought a bloody civil war before the country was a century old. And the regional tensions that overflowed then persist today in the Frostbelt-Sunbelt economic battles and what some who live beyond the Rockies see as Washington's "war on the West."

Canada, although settled by Europeans almost as early as the United States, did not become a confederation until 1867.

Today, the 23 million Canadians spread along the 3,200-mile band from St. John's to Ucluelet are struggling with centrifugal forces which make the regional tensions of the United States seem tiny ripples by comparison.

Those forces are not far below the surface in the election campaign which will climax Tuesday with the election of a new parliament and government in Ottawa. And it is the consequences of this campaign for the future of Canada's experiment in nationhood that gives the contest significance for all of North America, including the United States.

In other respects, it is not a very exciting campaign. Canadian journalists say the election had been so long anticipated when it was finally called that all of the arguments had long since been dulled by repetition.

The central figure in the election, win or lose, is Prime Minister Pierre Elliott Trudeau. The senior statesman of the western world in tenure and, admirers would say, talent, he is, at 59, showing the scars of 11 years in office.

There are still traces of "Trudeaumania" when he walks through the market square in a French-Canadian town like St. Hyacinthe, trailed by children and followed by the admiring glances of women shoppers.

But Trudeau, a moody and intensely private man, has become even gloomier and more withdrawn since the breakup of his marriage and the vast publicity surrounding his estranged wife Margaret's embarrassing memoirs.

His aides have gone through the campaign telling themselves that Trudeau "is finally getting his heart into the fight," which is their way of acknowledging that the zest for combat has just not been there.

Travelling with his challenger, Conservative Party leader Joe Clark, is, for an American political reporter, like taking a trip backward 20 years in time.

In the ballroom of a Holiday Inn here (a room that is small enough to make the Tory faithful in this predominantly Liberal territory seem plentiful), a 1950s-style band called "The People's Choice," which flies around the country with Clark, is warming up the crowd. The repertory ranges from country to cornball, from "Thank God, He's a Country Boy" (a tribute to Clark's roots in rural Alberta) to a sort-of-Dixieland stomp, "When Joe Clark Comes Marching In."

When the cheerleading ("Give me a C! Give me an L . . . .") is finally over, adn the real Joe Clark does come marching in, there is a palpable sense of letdown. The 39-year-old law school dropout and sometime journalist, a political staffer since he was 21, a member of Parliament since 1972 and leader of his party since 1976, is a plugger cruelling devoid of political charisma.

He makes up in earnestness what he lacks in wit. He praises the great "team" of Conservative candidate - almost all losers in these Liberal constituencies - on the platform behind him. He invokes the spirit of the perennial hockey champions, the Montreal Canadiens.Belaboring the point, he compares his candidates to the Montreal Expos baseball team, and says, "They cawin the pennant and so can we." If Montreal had a table-tennis team, you have the feeling Clark would work them into his spiel.

Now he is into his one big joke of the campaign. He says Trudeau had claimed God was on his side. "If He ever was," says Clark,"He's switching . . .and He's bringing His supporters with Him."

A big grin pushes his chipmunk cheeks even wider, and, as the crowd cheers on cue, Joe Clark throws both arms upward in an awkward V.*tRichard Nixon, vintage 1960, is alive and well and running for prime minister of Canada.

This is not another British election, posing clear alternatives on economic policy or challenging the basic direction of the party in power.

Inflation has been serious here, although at the moment it is less ruinous than in the United states. The Canadian dollar, which has been selling at a steep discount, is beginning to crawl back toward parity with the depleted U.S. currency.

Trudeau, who campaigned against wageprice controls in 1974 and slapped them on two years later, has slowly removed them. His basic approach in this campaign is to pretend that inflation and unemployment are myths.

Clark's contributions to the economic debate have been of comparable quality. While focusing on inflation as the main failure of the Trudeau government, he has promised a package of tax cuts and fresh government subsidies lavish enough for Liberals to label him "the $7-billion man," a figure they say would be the increse in the first Clark budget's deficit.

The contest between the troubled Trudeau and the untutored Clark (and the two minor parties, the socialist New Democrats and the Quebec-based populist Social Creditistes, who might win enough seats to deny either the Liberals or Tories a majority) could be put down to spring entertainment, were it not for the one serious question which hangs over Canada: Which, if either, of these men is equipped to preserve Canadian unity in the face of the challenge from the Quebec separatists?

The doubts about Clark start with the distribution of party strenth. The Tories historically have been the "English" party, and they go into this election holding only two of the 74 Quebec seats in Parliament. Nor is there much prospect for their adding to that total on Tuesday.

With so few Quebeckers in his party in parliament, Clark would have a hard time establishing a French "presence" in his government. There are widespread and apparently well-founded reports that supporters of Quebec Premier Rene Levesque and his separatist Parti Quebecois (PQ) are hoping for a Tory victory to increase the polarization in the country before Levesque's planned autumn referendum in Quebec on the first step toward separation.

A leading Trudeau supporter says, "We can't make the argument in public because it appears to be a threat, but I'm convinced that if English Canada rejects a French-origin minister on the eve of the Levesque referendum, it's the end of the country."

Deliberately or not, Clark had fed that fear by some of the things he has done in the campaign. His party is running a TV ad in Quebec, where Trudeau remains highly popular, accusing the prime minister of responsibility for a series of broken promises,turn-about and scandals.

In the final frame, the narrator pronounces Trudeau "coupable " (guilty), and there is the sound of a heavy door slamming shut. "The whole implication," says one of Trudeau's French-speaking lieutenants, "is that he belongs in jail, not in government."

Clark also raises eyebrows - and some Quebec hackles - with his comments on the unity question. After saying earlier that he would be prepared to negotiate a change in Quebec's status, he declared in mid-campaign that the referendum would have no influence on his government. "Quebec cannot vote itself out of Canada,"he said. The adamancy of that position was judged provocative even by some English-Canadian newspapers supporting him.

Clark has promised to put "a new face of federalism" that would make it more attractive for Quebec to remain part fo Canada, but it is not clear if that is merely a campaign slogan.

He has spoken of Canada as a "community of communities," and some of his admirers suggest that his own background in Canada's small-town West may give him a greater sympathy for the desire of Quebeckers to preserve their own rural culture than the cosmopolitan Trudeau can pretend to muster.

Clark also notes that he is an instinctive consensus-seeker and that his personal links to the other provincial premiers - all but three of whom are fellow Conservatives - could help find a formula for redesigning federal provincial relations on a basis more acceptable to Quebec.

"Trudeau has boxed the compass on this issue and has nothing more to offer," says one Montreal commentator, who is opposed to separation. "Joe Clark may not be too bright, but at least his election would break the present impasse and open the possibility of new discussions. If concessions must be made by English Canada to keep Quebec in the confederation, it is logical a prime minister from English Canada could engineer them."

But that may be a dim hope. Marcel Rioux, a University of Montreal sociologist and adviser to Levesque, says, "It is no secret that the PQ leaders, including Levesque, want to get Trudeau out of there." But,asked if Clark's "community of communities" concept appeals to him as a vehicle for Quebec's aspirations, Rioux is scornful. "All he is talking about is bringing a few feathers and a bit of color to the gray mass of the dominant culture," Rioux says. "He is a Calgary man, and Calgary, for all practical purposes is an American city."

If Joe Clark is a dubious bet to defuse the separatism of French Canada, there is no guarantee that Trudeau can do better. Separatism has gained strength during his 11 years as prime minister, despite his strong support for bilingualism and biculturalism. Rioux is far from alone in describing Trudeau as " very rigid" in his preference for a strong federal government.

Indeed, Trudeau has made the keynote of his campaign the claim that he represents the "strong leadership" necessary to hold Canada together against the centrifugal forces of regionalism, while Clark, he asserts, would bend and break in that kind of storm.

But there are many skeptics in Quebec, including members of Trudeau's own party. Claude Ryan, the leader of the Quebec Liberals and a sharp critic of Levesque's separatism, says that "neither of the two men (Clark and Trudeau) has responded satisfactorily to the ferment in Quebec. While Ryan is supporting Trudeau and the Liberals in Tuesday's elections, he does not buy the argument that their defeat would be the last straw for Quebeckers.

The PQ would try to portray Trudeau's defeat as a slap in the face for French Canada." Ryan said. "But I think the majority of Quebeckers would regard the defeat of the Trudeau government as a criticism of its entire record, not a referendum on the specific question of national unity."

Meantime, Trudeau faces a problem that is the mirror-image of Clark's lack of Quebec deputies. The prime minister has lost most of his prestigious cabinet colleagues from populous Ontario and has virtually no backers in Parliament from the western provinces.

Trudeau is criticized, even by his own aides, for his tendency to run a one-man government and for letting the strongest Liberal spokesmen for English Canada leave the cabinet without finding adequate replacements.

With Toronto the commerical hub of a province that represents more than one-third of Canada's population and an even larger share of her wealth, the neglect by Trudeau is costly.

As for the western provinces, they have turned away from Trudeau's goverment almost as massively as Quebec has rejected the Conservatives. Only 11 of the 67 seats in the last parliament from the four western provinces were held by Liberals, and, again, there is little prospect for a change Tuesday.

Those provinces look to the United States for markets and supplies, more than they look eastward to Toronto and Montreal. And the Montreal Star commented editorially that whatever the threat in leaving Quebec unrepresented in Ottawa, "it could be almost as dangerous for national unity to perpetuate the isoltion of Western Canada from power in Ottawa."

In that sense, Tuesday's election is almost certain to leave Canada still confronting the tensions that tears at its national unity. The lessons for the United States of the dangers of disunity, in the absence of truly national parties, are very clear. CAPTION: Illustration, Pierre Trudeau. By Kerry Waghorn