In a nationally televised address tonight, Prime Minister Brian Mulroney attempted to sell an increasingly wary Canadian public on the virtues of opening up the trading relationship with the United States.

But, Mulroney said, if he determines that trade talks between the two nations, which are to begin formally in Washington on Tuesday, will not lead to an agreement favorable to Canada, he will not hesitate to back out of negotiations.

"Canadians have every right and reason to ask what's in these talks for us," Mulroney said. "The answer is more jobs and greater prosperity. If there is any doubt at all in that regard, there'll be no deal."

The 15-minute speech, broadcast from the prime minister's office in Ottawa, marked the first time during his 20 months in office that Mulroney has sought nationwide air time to directly address Canadians on any topic. Although he has made the free-trade negotiations the centerpiece of his foreign and economic policy, his previous remarks have been largely confined to partisan exchanges with opposition critics in the House of Commons.

Although oil- and timber-producing provinces as well as many Canadian business groups have endorsed the trade talks, labor unions, industrial provinces and artists and intellectuals who oppose them have been the loudest voices in the debate here so far.

For the most part, Mulroney has attempted to pitch a new trade agreement as simply "a good idea" without going into detail about pros and cons. Government-commissioned studies on the potential effects of free trade with the United States have been heavily excised before being made public, particularly sections dealing with the number of jobs that might be lost if barriers are lowered.

In his speech tonight, Mulroney said his government would "be most sensitive" to industries that need help adjusting. He also sought to appeal to Canadian pride, saying Ontario petrochemical workers, Montreal tailors, Winnipeg bus manufacturers and other workers are all capable of competing head to head with their American counterparts.

The prime minister also made it clear that trade barriers erected by independent-minded Canadian provinces will be overhauled.

"As Canadians, we must put our own house in order," he said. "We must remove barriers to interprovincial trade. For example, why can't you use Ontario bricks on a Quebec construction site and why can't a Montrealer buy a bottle of beer brewed in the Maritimes?"

Recent U.S. protectionist measures against Canada have strengthened Mulroney's standard argument, which he reiterated tonight, that a new trade deal with the United States is essential to secure Canadian access to U.S. markets.

Leaders of the two parliamentary parties, who were given television time to respond to Mulroney's speech, continued to question whether the government was competent to defend Canada's interest in tough negotiations.

"Taking on a country 10 times stronger than we are in trade negotiations holds great risk for us," said Liberal Party leader John Turner.

Both Turner and Ed Broadbent, leader of the New Democratic Party, also riticized the timing of the discussions, which begin during an election season in the United States when protectionist sentiment is strong.

Both of them also questioned whether Canada's political independence and "cultural sovereignty" might be bargained away.

"We are Canadians and we don't want that negotiated away," said Turner. "We Canadians have built a unique community in North America. We like it here. We like our system of government, our spirit of tolerance, safety on our streets, our way of doing things and we do not intend to become the 51st state of the American union."