One bit of information missing from William Claiborne's article "After Two Years Most Canadians Still Distrust Trade Pact With U.S." {news story, Jan. 5} would have put Canadian suspicion and antipathy toward the trade agreement with the United States in clearer perspective. A reader might assume that since Prime Minister Brian Mulroney's party won the last election and since, as Mr. Claiborne stated, it "in effect, was a referendum on free trade," that the majority of the electorate had been in favor of the agreement and that two years had reversed this original approval. That would be misleading.

Although Mr. Mulroney's party did win a decisive parliamentary majority in that election, it did not secure a majority of the popular vote -- that was secured by the two other political parties, both of which opposed the pact. This was possible because the opposition to the ruling Conservatives had been divided, and the Canadian electoral system elects the person who comes in ahead of the others in each single district.

The system works best with two parties, tolerably well with a weak third party, but less well with a moderately sized one, as is the case in Canada. Thus Mr. Mulroney actually lost the referendum while winning the election.

Although by the rules of parliamentary government a victorious administration has every right to enact the program on which it was elected, Mr. Mulroney was definitely taking a political risk in doing so and may eventually pay a political price.

HARRY LAZER Emeritus Professor of Political Science City University of New York New York