The clearest issue to emerge in Canada's current election campaign is a credibility problem of the two key cotestants.

After 11 years in office, Liberal Prime Minister Pierre Trudeay, 59, is no longer a swinging and youthful leader offering a new vision to the country or new alternatives for resolving the gravest constitional crisis in its history.

His Conservation challenger Joe Clark, 39, has yet to demonstrate to the voters both his strength and his ability.

As a Toronto Globe and Mail columnist put it Canadians "don't believe Trudeau will do what he says he will do. It Clark's case, they don't believe he can do what he says he will do."

Apart from the personality question, key issues facing Canadians in the May 22 vote involve the thorny problem of national unity and an ailing economy. But the initial rhetorical salvos suggest differing electoral strategies of the two men.

Trudeau's basic campaign thrust is to present himself as the only man who can keep English Canada and French Canada together in one country while he tries to avoid the economic issue on which is he is more vulnerable.

Clark who does not seem to have a clearly defined program for dealing with the threat of Quebec secession, is making the economy the key issue. He is charging that Liberal mismanagement is the cause of the current 9 percent inflation, 8 percent unemployment and the decline in the value of the Canadian dollar, which has now worth only 85 U.S. cents, a slide of over 20 percent in only three years.

Several public opinion studies published in recent weeks found that a majority of Canadians are most concerned with the programs of inflation, unemployment and economic stagnation.

But the unity issue coupled with widespread uncertainties caused by the prospect of an independent Quebec cut deeply and may yet bring into sharp focus the relative ambigutiy of Clark's views on the matter.

In an effort to sharpen this issue, Trudeau has suggested that anyone who does not attach importance to national unity is "almost treasonable to Canada." As the Liberal Party leader frokm French-speaking Quebec, he is projecting himself as the key opponent of the secessionist Parti Quebecois, which has controlled the government of his native province since 1976.

Four months ago, Trudeau was 10 percentage points down in the Gallup poll and his Liberals were in disarray, having lost 13 of 15 by-elections in October.

By February, however, Clark's Progressive Conservatives' lead narrowed to only two percentage points following a high profile 12-day world tour by Clark. The tour was plagued by logistical disasters, missing luggage, strange verbal distortions and physical awkardness that included Clark's walking directly into a bayonet-wielding soldier in the honor guard he was inspecting on the Golan Heights. Canadian tropps serve with U.N. forces keeping peace between Israel and Syria.

Media coverage of the tour raised doubts in Canada about the relatively young Albertan's ability to lead a government.

Latest polls suggested the two main parties are running neck-to-neck, raised the possibility that the small leftist New Democratci Party may hold the balance of power in the new House of Commons.

A decisive vote on national unity would come in a referendum to be held in Quebec later this year when the Parti Quebecois government will seek a mandate to negotiate sovereignty for Quebec in association with the rest of Canada.

The May 22 federal election will have a direct bearing on the Quebec referendum. If Trudeau wins, he is likely interpret the vote as a mandate to halt the steady drift toward disintegration.Such an outcome could complicate Quebec separatist efforts since it would reflect English Canada's determination to maintain national unity.

Clark's victory, on the other hand, would be seen as increasint polarization between English and French-speaking Canadians, although some observers argue that such an outcome would be more conductive to a negotiate settlement with Quebec.

The crucial area of contest will be Toronto and other areas of Ontario, the largest of Canada's 10 provinces. The Liberals are strongest in Quebec while the Conservatives virtually dominate the Western provinces.

Trudeau's popoularity in Ontario has dropped over the past month largely because of increased doubts about his lieadership, but also because of personal dislike for him.

Moreover, he is being blamed for the development of regionalism that came to the fore during his administration. Apart from the resurgence of Quebec separatism. English Canada is unable to provide a unified response to any of the major issues facing the country. This is especially true of Western Canada with its growing economic power.

One of Trudeau's accomplishments early on was the passage of the Official Languages Act, which gives the French equal status with English in government services. It was accompanied by his plan to make Canada 'irreversibly bilingual' involving massive training in French of English-speaking civil servants.

These and other linguistic measures promoting French have caused resentment in English Canada and Trudeau has since acknowledged that "bilingualism" was not meant to make all Canadians speak both languages. CAPTION: Picture 1, JOE CLARK; Picture 2, PIERRE TRUDEAU