They were singing "Oh Canada" last week, Pierre Trudeau on the stage and more than 10,000 Liberal Party faithful packed in a downtown plaza, when a small plane pulling a large sign reading "Goodbye Pierre" began circling overhead.

Trudeau looked up at the sign and smiled. Then his brow lowered and his eyes became icily intense.

"I must remind you of Mark Twain," he declared, "When he said that the news of his demise is some what exaggerated."

The news of a Liberal defeat, proclaimed by local columnist and pollsters here a week ago, may be premature. In the closing days of the campaign leading up to Tuesday's national election, Trudeau finally has come into his own.

There he was alone on the stage the man with a face reminiscent of an Iroquois brave with that vital spark of charisma turned on fighting for a strog Canada and calling his Conservative opponent Joe Clark a "joke."

After weeks of lackluster performance, the old magnetism was there, fueled by hopes that everything was not lost, that he can win a fourth term.

Yet after 11 years as prime minister, Trudeau is a changed man leading a changing country. Both have come a long way since the euphoric days in 1968 when he was first elected.

At 59, Trudeau still projects vigor and zest, but the love-in, known here as "Trudeaumania" has turned into a love-hate relationship.

Canadians recognize his skills and his dominance of the political scene and they take a vicarious pride in his high reputation around the world.

Yet, simultaneously, they dislike himf or it. As a successful politician he has become less open and more cynical, wilier and harder. He has squandered his political capital with his aloofness and bursts of immature and smart-alecky behavior. His flamboyance, the Gallic flare and French rationalist mind rubbed many people the wrong way.

There is an underlying mood for change is an underlying mood for change in Canada which he has sensed earlier. He had wanted to retire from politics, he mused in an interview last night, and was considering it seriously before the separatist Parti Quebecois won a stunning victory in his native province of Quebec in 1976.

It had been his desire to dispel Quebec nationalism and replace it with full acceptance and involvement of 6 million French Canadians in Canada's national life that brought him into politics in the first place.

His wife Margaret wanted him to retire and devote himself to her and their three small children. The prospect of Quebec sovereignty, however, meant a possible breakup of Canada and the destruction of his dream.

It had come down to the choice between his marriage and his political career, he told friends. He sacrificed his marriage out of a sense of obligation.

"I suppose you could say that I could get out while I'm at the top," he said last night. "I'm in politics still for the reason I got in in the first place: Because I think at this time in the life of our nation and in our lives there are certain directions that should be given to Canada and politics is the way to do it.

"Particularly this time with the referendum coming up in Quebec, I just feel that if I walked away from this one I wouldn't be able to face myself in the years to come."

It is Trudeau's deep conviction that as a Quebecer in Ottawa, he would be able to stem the tide of Quebec's separatism and that the election of Clark would bring about a polarization between Canada's two main ethnic groups that could only strengthen the separatist cause.

He wants to see that Quebecers, "When they are going to vote in the referendum, have a feeling of being a part of Canada." He feels tnat Canada's national life is entering its most decisive phase on grounds he himself has prepared and that both the country's fate and his place in history will be decided in the coming months.

It has been a very difficult year for Trudeau, both politically and personally. His standing in the pools has been low and he had sought to maneuver forces and strengthen the economy suffering a 10 percent unemployment, 9 percent inflation and a weakening dollar before calling the elections.

The memoirs of his estranged wife Margaret came out just as the campaign was getting under way. He has never discussed it except with closest friends, but for an intensely private person such as Trudeau it was a blow.

The breakup of his marriage to a much younger woman has left him with a sympathetic image of a single parent caring for his children. Margaret's stories about sex, drugs and other men turned the public against her.

Yet her book, a person close to Trudeau said, assaulted his deepest emotions.

"To have his sense of privacy repaed like that, he literally felt stripped naked in front of the Canadian public," the friend said.

Trudeau's first two weeks of campaigning were a disaster. Hecklers shouting obscenities about his wife frequently put him off balance. He never responded to them. Instead he used intemperate language in attacking the farmers, the unemployed and others.

But gradually he came into his own. The old charm and the keen edge of his intellect began to tell. It was an uphill struggle against the desires for change after 11 years of Trudeau and 16 consecutive years of his party in power.

The major goals of his policies have been elusive. His "third option" policy - an attempt to reduce Canada's dependence on the United States by increasing trade with other countries - has failed to produce significant changes.

The passage of the official languages act making Canada "universally bilingual" has had modest success but also has created hostility toward him in western Canada. The once robust economy has deteriorated sharply.

Perhaps the most serious difficulty was his credibility. He fought the last election in 1974 by campaigning against controls on wages and prices as advocated by the Conservatives. Once reelected, Trudeau imposed them for three years.

Only in the last two weeks has his campaign begun to pick up strength, mainly because of public doubts about Clark's ability to lead the country. Yet the polls in the crucial Ontario area were still bad, showing the Liberals 10 points behind.

Last week, however, news was somewhat more encouraging. The Conservatives became increasingly worried here in Toronto as the gap began to narrow.

The modest upturn in fortunes has cheered Liberal forces and made Trudeau campaign fiercely in these last two days. Yet even now, in this crucial period apart from his consistent attempts, Trudeau is conducting his campaign on his terms.

He had apparently decided to fight it on principle even at the risk of losing. He finally began to speak about his record, reminding Canadians that their country has done resonably well in an increasingly difficult world, that Canada's inflation is still lower than many other Western countries and that unemployment, while high, is cushioned by a very generous unemployment compensation system.

He has also talked about a larger problem of the West and about the failing produced by expectations to "take out of the society more than you put into it."

The impact of the oil crisis and the shift in distribution of financial might has to be accounted for, he said.

"We don't have that money anymore and to that exent we have become poorer," he said. "It doesn't mean that the future ahead is dismal or gloomy. But it does mean that we will have inflation if we try to pretend that this oil crisis didn't happen."

Trudeau believes the Western world is entering a difficult period and that Canadians will have to lower their expectations. He cannot hide his contempt for Clark who talks about a "country of literally unparalleled opportunity" and confidence in the future."

Yet the campaign has produced no evidence that the public wants to hear admonitions about diminishing expectations. In fact, Clark's proposals to reduce taxes and make mortgage interest payments tax deductible is perhaps the most popular issue among the middle class.

In the end Trudeau has become the Liberals' biggest asset and heaviest liability. The election will be a plebiscite on Trudeau and his vision of the country. His personality, his forthrightness, the limits he had set on personal concessions to court public favor or what people here call his "arrogance" will in no small measure determine the outcome of Tuesday's vote. CAPTION: Picture, Prime Minister Trudeau is mobbed by supporters at Hamilton, Ontario, rally. UPI