With the Canadian Parliament due to begin debating the free trade agreement with the United States today, a supporter of the pact in the U.S. Congress warned yesterday that trade relations between the two nations will worsen if the pact fails to win support from both legislatures.

"It will be all downhill" if the agreement is rejected by either Parliament or Congress, said Rep. John J. LaFalce (D-N.Y.). "I don't think relations could revert to the status quo" because "expectations have been raised so high."

LaFalce and Sen. Daniel J. Evans (R-Wash.) sponsored an all-day Capitol Hill symposium on the pact, which will create the world's largest free trade area. The agreement, reached in its final form last Monday, will be signed by Prime Minister Brian Mulroney and President Reagan on Jan. 2.

LaFalce predicted that the agreement will win approval from Congress, which will take it up next spring under special provisions that permit neither amendments nor delaying tactics.

The issue has not aroused much controversy in this country. In Canada, however, the pact has ignited emotions over questions of sovereignty and nationalism and concerns that it would lead to Canada becoming the 51st state.

The two Canadian opposition parties -- which have gained equal standing in the polls, though not in Parliament, with Mulroney's ruling Conservative Party -- have vowed to overturn the agreement. In addition, three of 10 provincial premiers have come out against it.

LaFalce warned of "serious repercussions" if the opposition gains power in Canada and "rips the agreement up." He suggested that Congress will make sure that Canada is included in any future efforts to tighten U.S. laws against other countries' unfair trading practices.

Although he predicted congressional passage for the trade pact, LaFalce said he was concerned that supporters of the agreement have not yet emerged as a major lobbying force. "The only voices Congress will hear are those interests with axes to grind that are opposed" to the agreement, he said.

John McDermid, parliamentary secretary for Trade Minister Pat Carney, said at the conference that the Canadian House of Commons, where the Mulroney government has an overwhelming majority, will vote on the agreement by the end of the week. Then the government will draft legislation putting the terms into Canadian law.

Despite the opposition, McDermid declared that the Mulroney government "is committed to this agreement. It forms a very important part of our economic plan for Canada."

The pact was also supported by Donald MacDonald, a Toronto lawyer and former Cabinet member in the Liberal Party government. He headed the Royal Commission that recommended to Mulroney that he seek a free trade agreement with the United States.

Paula Stern, former head of the U.S. International Trade Commission, praised the agreement, which she said would produce "real payoffs" in the eliminations of tariffs within 10 years between the world's two largest trading partners.

But Bob Rae, leader of the New Democratic Party in Ontario and head of the official opposition in the provincial legislature to the ruling Liberal party, said "there is no consensus" in Canada for the free trade pact, which "has such an impact on many aspects of Canadian sovereignty."

"In my view," he said, "we've given away the store."