Pierre Trudeau, who quit politics with a tearful show of emotion three months ago, seems to be heading toward one of the most spectacular comebacks in Canadian history.

On the eve of Monday's general elections, posters, polls and political observers are predicting a significant shift toward Trudeau's Liberals. His camp is jubilant. And the 60-year-old former prime minister has run too many electoral races not to know how to break into a sprint as the finish line nears.

As a result, Trudeau has abandoned his diffident and heavily programmed style of the past eight weeks to make boisterous statements about the future, including a pledge that his government would stand up to the United States in defending Canadian economic interests. He also left the door open for Canadian participation in the Summer Olympic Games in Moscow.

Buoyed by the election forecasts, the Liberals have changed pace in the campaign's closing days. Until last Monday, Trudeau was limited to reading one speech that said little except how incumbent Conservative Prime Minister Joe Clark has misused power.

But during the past week the speech was abandoned in favor of extemporaneous remarks sprinkled with flashes of temper and animated give-and-take debates for which Trudeau was renowned as prime minister from 1968 until his defeat last May by Clark.

In addition, for the first time Trudeau has told Canadians how he would use power if he regains it on Monday.

In a speech yesterday, he vowed that he would stand up to the United States to make sure that Canada gets "its fair share" from the multibillion-dollar automobile industry largely owned by U.S. interests. He said the Americans have been beneficiaries of a U.S.-Canadian auto pact under which they sold $2.9 billion more in cars and parts in Canada last year than they purchased from Canada. He called this a disgrace that "must be changed."

Speaking in Toronto a day earlier, he signaled voters that he will interpret a victory Monday as a mandate for a slight shift to the left and toward economic nationalism.

His government, Trudeau said, would be "an active participant" in economic development and would "expand Canadian ownership and control" over this country's industrial and national resources. He said he would seek "new instruments" to curb "foreign ownership" -- a term applied to U.S. interests here, which amount to about $50 billion.

The Liberal leader, in another effort to distance himself from what is perceived as an overwhelmingly pro-American Conservative government, also hinted that he may reverse Clark's decision to boycott the Moscow Olympics.

Such a boycott, Trudeau said, could be effective only "if there is a massive participation not only by Western nations but also by Third World nations." He said he did not foresee such massive support for a boycott.

For the first time in the campaign Trudeau also expounded on his vision of a united Canada led by a strong central government -- the issue on which he was defeated last May. And it is this issue that goes to the heart of Canada's uncertainties about its future threatened by a secessionist drive in the French-speaking province of Quebec.

The Conservative government has sought to defuse the Quebec issue by speaking of Canada as a "community of communities" and by indicating willingness to devolve authority to the 10 provinces to combat regionalism and separatism.

"Canada is not a community of communities, it is one nation," Trudeau said Friday. "Canada is a country where people of all backgrounds can meet in a civilized way, held together by a rope to form a strong nation and by a strong central government."

Should Trudeau win -- and if he does not, mass embarrassment among sheepish pollsters can be expected Tuesday morning -- he clearly intends to interpret the vote as a mandate for his policies, which Canadians had rejected only last year.

The Gallup Poll published yesterday gives the Liberals 48 percent of the decided votes, compared to 28 percent for the Conservatives and a surprising 23 percent for the socialist New Democratic party. The poll indicated weakening Conservative strength in the past week.

A CTV television poll published today showed a smaller gap of 10 points. The poll gave 43 percent to the Liberals and 33 percent to the Conservatives.

It is increasingly apparent that Trudeau's new exuberant mood reflects his expectation of political vindication. After he was beaten in the last election, he announced in November that he would step down as leader of his party and that he intended to devote his time to his three young children. They have been living with him while trudeau's estranged wife, Margaret, pursued other interests in the United States, Europe and Canada.

But all that changed a few weeks later when Clark was brought down in the House of Commons over a belt-tightening budget that included a proposal for an 18 cents per gallon tax increase on gasoline.

The Liberals, who were involved in their search for a new leader, were caught by surprise and forced to call on Trudeau to come back. He did, and largely on his terms.

But Trudeau did accept the party's election strategy, which sought to avoid reminding the voters why they voted against him last May.

As a result, he abandoned his gunslinger image and his combative tone. Until last Monday he was not only vague in issues but he also read prepared texts line by line with the glassy-eyed concentration of an inexperienced newscaster scanning his tele-prompter.

For the same reason he stayed away from hot-line shows, refused to debate Clark, and held only a couple of press conferences. The thrust of this seemingly cynical strategy was to focus public doubts on Joe Clark's image and make that the main issue in the campaign.

Judging by the polls, the strategy worked. The fact that he abandoned it last week reflects not only Liberal confidence but also Trudeau's signal that he is seeking not just a victory but a mandate to give his anticipated victory direction and legitimacy.