During his last few months in office, President Reagan has little chance of achieving anything major that requires enactment by a Congress controlled by his opponents - with one big exception.

That exception is the free-trade agreement that the administration has negotiated with Canada. Congressional leaders have agreed to seek a vote on this pact by June. If it is approved, it will rank as one of the great economic reforms of the 1980s.Utah stands to benefit from the agreement. In 1986, Utah and Canada traded over $399 million worth of commodities. Utah, however, sells to Canada more than two and a half times as much as it buys from that country. The U.S. Department of Commerce estimates that, with the expansion of government procurement opportunities through the new trade pact, Utah firms will be eligible to compete for an additional $500 million in Canadian federal public sector purchases alone.

The agreement would phase out all tariffs between the two countries over a period of 10 years - turning the entire area between the Rio Grande and the Arctic into one gigantic free-trade zone, by far the world's largest. In a world tantalized by breathtaking revolutions in trading opportunities but threatened by short-sighted provincialism and protectionism, it would be a stunning breakthrough and a model for the future.

North Americans on both sides of the border would benefit twice from the agreement - as consumers and as workers. By ending tariffs which average about 10 percent for U.S. goods sold in Canada and 5 percent for Canadian products entering the United States, it would lower retail prices on a wide range of items. By opening new markets to producers it would stimulate investment, profit - and payrolls.

Scripps Howard News Service quotes William Lilley of the American Business Conference, an association of high-growth firms, as estimating that the pact would add as much as $17 billion a year and 750,000 new jobs to the U.S. economy. And that is just the short run.

What's more, the U.S. would be assured of secure access to Canada's vast energy resources, which can be particularly valuable if other foreign sources are jeopardized. Under the agreement, Canada is to provide the U.S. with non-discriminatory access to oil, gas, and electricity at essentially the same prices Canadians pay. This can save American consumers billions of dollars if there is another oil shortage.

If they care about about competitiveness and prosperity, America's lawmakers should vote for this agreement with enthusiasm and without delay.