As he roared across the hustings over the past four weeks, Prime Minister Pierre Trudeau attacked his opponent so hard that he seemed like the hungry political challenger rather than an incumbent with 11 years in office.

This reversal of roles in Canada's election campaign lends credence to the widespread belief that Trudeau is waging an uphill battle for a fourth term.

With his jaw thrust ou, with his accusing finger pointing in all directions, the 59-year-old Liberal prime minister was clearly taking a gamble with his hardhitting approach.

His Conservative challenger Joe Clark, by contrast, has campaigned so serenely that he almost looks like a prime minister already assured of re-election. The 39-year-old Clark was risking little as he talked about negotiation and conciliation.

The reversal of roles, apart from reflecting the distinct possibility that Trudeau may be defeated on May 22, indicates that the personalities of the two men have emerged as the clearest issue facing the country's 13 million voters.

The latest Gallup poll put the two major parties at 41 percent each. But political experts here say that this result gives a slight edge to Clark since Liberal strength is concentrated in the French speaking Quebec while Conservatives are ahead in English Canada, which has three-quarters of the seats in Parliament.

Not only has Trudeau been hammering Clark, but he also lashed out at the unemployed, farmers and other critics who do not share his view that national unity endangered by Quebec's secessionist movement is the most important issue in the campaign.

Trudeau's behavior is believed to stem in part from a great emotional strain caused by his estranged wife Margaret's memoirs, "Beyong Reason." The publication of the book, which includes her tales of drugs and sex and reveals details of the Trudeau's marital difficulties, coincided with the first phase of the campaign. Trudeau is known to has been deeply wounded by the book.

After someone shouted an obscenity about his wife as he was entering a campaign rally in Vancouver, Trudeau delivered a hard-hitting speech berating demonstrating unemployed, telling them to stop drinking and find a job.

"Come on, there are people here who are working and they're doing an honest day's work, they're not just being paid to carry a sign around like you are. Get off your ass. Your want a job? Well stop drinking and you'll get a job."

Later this week talking to farmers in Quebec, Trudeau attacked them as "chronic grumblers."

"Sure farmers are guys who grumble by profession.They grumble about temperature when there's sun and they say there's too much, when there's rain they say there's too much. A farmer is a grumbler, he's happy when he grumbles."

These remarks and other similar outbursts by Trudeau have been used by Clark to his own advantage.

Clark's campaign echoes that of Jimmy Carter's in 1976. While avoiding any precise definitions and commitments, Clark has been saying that Canada needs a government as decent and as compassionate as the Canadian people themselves. And his basic promise is that a Conservative government would restore Canadian's sense of pride.

Trudeau's highly publicized attacks have taken attention away from his basic campaign theme: that Canada needs a strong central government to face Quebec secessionists and the demands for more power from western provinces. His combative style was in part designed to show Trudeau as the country's only leader strong enough to steer Canada through tough times ahead. In every speech he attacks Clark as a kind of "headwaiter" for power hungry provincial premiers.

Clark talks little about national unity and the possibility of Quebec's secession, saying only that Trudeau has ruled by confrontation rather than conciliation. Instead Clark focuses almost exclusively on economic problems such as Canada's 8 percent unemployment, 9 percent inflation and a dollar that has declined 16 percent in value against the U.S. dollar since 1977.

Clark's low-key style grows from a conviction among his strategists that Trudeau, after 11 years in office, is himself the chief campaign issue and that by avoiding any serious mistakes the Conservative Party will win.

Both sides have advanced various schemes to ease the tax burden of Canadians, but these are poorly defined and are viewed largely as campaign promises.

Ironically the leader of Quebec's secessionists, Premier Rene Levesque, who precipitated the current national unity crisis, is not participating in the debate. Levesque is on a honeymoon in southern France, a gesture his supporters describe as symbolizing French-Canada's indifference to Canadian politics.

There are growing indications here that neither of the two major parties would win majority in the next Parliament and that the small Socialist New Democratic Party may emerge holding the balance of power.

The cynical view at this half-way point in the campaign was expressed by Peter Newman of the national weekly magazine Maclean's. He said the most beneficial outcome would be a Conservative victory that would force Trudeau's resignation "to be followed shortly by another election that would bring about a Liberal minority triggering Joe Clark's departure.

"Then we could start all over again. Meanwhile the nation's voters can only hope that one of the two hoofers due to form the government will wake up to some higher ideal than capturing or retaining power." CAPTION: Picture, Trudeau: "A farmer is a grumbler, he's happy when he grumbles." AP