IN WASHINGTON, WHERE ALL THINGS are possible, the Canadian trade negotiations have suddenly moved to the front burner. The Americans are protesting that they never were anywhere else. And the Canadians, who complained loudly of being taken for granted, now are openly optimistic.

Canadian Prime Minister Brian Mulroney's sharp words, when he was visited earlier this year by Vice President George Bush and Clayton Yeutter, U.S. trade representative, are probably most responsible for the change. But so is the political shift in Congress caused by the Democratic victory last November. Sen. Lloyd Bentsen, the Texas Democrat, made a visit to Ottawa one of his first priorities even before he actively assumed leadership of the important Senate Finance Committee. The senator has made no secret of the fact that he places a favorable settlement of outstanding issues between the United States and Canada high on his agenda.By now, it may also have occurred to those in Washington concerned with such weighty matters as the General Agreement on Tariffs and Trade negotiations that what evolves between the two North American nations could have an important bearing on what happens at Geneva. Specifically, the Canadians are seeking:

* Assurance that whatever is given, in terms of access to the U.S. market, will not suddenly be taken away. In the past, the Canadians complain, the United States has behaved capriciously in closing the door to U.S. imports of Canadian shakes and shingles, potash, uranium and potatoes, among other items.

* Much more precise definition of what constitutes subsidies and market prices, to avoid disputes over anti-dumping triggers and countervailing duties.

* Far more binding procedures for arbitration when such disputes do arise.

* Complete openness in bidding for public sector procurement.

These are largely procedural concerns, but they bear importantly not only on U.S.-Canadian trade relations but on this country's relations with its other trading partners. Whatever is agreed upon between Washington and Ottawa, thus, might well serve as a model for what might be expected from the GATT.

Things are going better north of the border, but there still are unnecessary flare-ups. Prime Minister Mulroney, for instance, felt compelled to chastise Mr. Yeutter when the latter proposed that publishing and broadcasting, among other cultural industries, be placed on the bargaining table.

If he wasn't aware of it before, Mr. Yeutter soon became apprised of how sore a toe he had touched. The prime minister said his proposal displayed ''stunning ignorance" of Canada and was "completely insensitive and totally unacceptable to the government of Canada."

In law school, students are required to engage in dialectic advocacy, a process of placing one's self in the other guy's shoes - each of the other guys, plaintiff, defendant, key witnesses, friends of the court. Only after doing this homework thoroughly are they considered ready to represent a client.

It might be useful if U.S. negotiators, not only on the trade front, were required to adopt some similar practice.

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