Distrust of the United States and fear of American domination is fueling a groundswell of opposition to a U.S.-Canada free-trade agreement and to the ruling Conservative Party that sponsored it, economists say.

Public opinion polls released in the past three days indicated the opposition Liberal Party's crusade to block the agreement is gaining strength, while the Conservative government of Prime Minister Brian Mulroney is losing voter support.Leading economists said Monday the free-trade agreement will collapse unless the Conservatives can find a way to sell the treaty to the public and win a majority of seats in the House of Commons in the Nov. 21 general election.

A poll for the Canadian Television network released late Sunday showed 39 percent of Canadians planned to vote for the Liberal Party, compared to 35 percent for the Conservatives and 23 percent for the socialist New Democrats.

A poll released one day earlier by Angus Reid Associates Inc. gave both the Liberals and Conservatives 35 percent of decided voter support and 28 percent for the NDP.

The Angus Reid poll also confirmed an earlier poll indicating mounting opposition to the agreement. The poll found 54 percent of Canadians oppose the initiative, 35 percent support it and 11 percent are undecided.

The trade agreement would take effect Jan. 1 if the Canadian Parliament approves it before that date. But approval is likely only if the Conservatives win a majority on Nov. 21, because both opposition parties have taken a stand against the treaty.

The agreement, which has been approved by Congress and signed by President Reagan, would dramatically alter the way the United States and Canada do business with each other.

The pact would establish a bilateral panel to hear trade-related disputes and the countries would have up to seven years to set new regulations regarding government subsidies for businesses. Opponents point out the agreement also removes restrictions on American banks operating in Canada and allows takeovers of Canadian companies by American firms.