President Reagan and Canadian Prime Minister Brian Mulroney have a lot in common. They lead huge free countries that are neighbors, both are rock-solid conservatives, and they like each other personally.

In addition, their political fortunes are similar. Each has won by a landslide, each has seen a comeback by legislative opponents, and each has been dismayed by politic scandals among aides or former aides.Despite all of these similarities, the prime minister's two-day visit to Washington this week is bound to have some sharp edges.

Two of Mulroney's most cherished projects the U.S.-Canada trade agreement, and the Reagan pledge to do something about U.S.-caused acid rain that is killing Canadian forests seem to have fallen victim to inaction.

The free-trade agreement, popular in Canada and helpful to Mulroney, is bogged down in Congress because of a battle between Reagan and Congress over other trade issues.

There is nothing Mulroney can do about that, but if support for the trade pact fails to materialize in Congress, it could politically hurt the Canadian prime minister, who is thinking of calling an early election next fall after four years in office.

The second issue acid rain may provide more of a confrontation. It is an emotional topic in Canada. Wind- carried pollution, usually blamed on U.S. coal-fired power plants in the Midwest, is damaging lakes and forests in eastern Canada.

When Reagan and Mulroney met in Ottowa last year, the president promised to negotiate a treaty on acid rain. Yet when Reagan returned to Washington, nothing ever came of the pledge. It's not something Mulroney is going to let the president forget, nor should he.

Acid rain is produced by nitrogen oxide and sulphur dioxide pollutants. It is carried hundreds of miles by winds in the upper atmosphere, combining with moisture to produce nitric acid.

Coming down as polluted rain, it has killed thousands of trees, not only in Canada, but the Northeast U.S. as well. It also has polluted rivers and inland lakes in Canada, destroying fish and plant life.

Acid rain is an issue that can't be responsibly ignored, for America's sake as well as Canada's. A new report says it may be contributing to a decline of marine life in U.S. Atlantic coastal waters.

The meeting with Mulroney offers a perfect chance to focus more attention on a problem that will not simply go away. Sooner or later it will have to be faced. And sooner is better.