Toronto — The Liberals are getting restless. The party that has ruled Canada for all but 21 years of this century is in a serious decline, and many Liberal backbenchers are beginning to blame their troubles on Prime Minister Pierre Trudeau.
This past week, six Liberal members of Parliament called for a leadership convention to get rid of Mr. Trudeau. Their hope is that a new leader can remedy the dismal record in the polls. Without some kind of turnaround, many of these same backbenchers stand to lose their seats in the next parliamentary election, which has to come in the next year and a half.
''The prime minister is going to have to resign. The people want him to retire,'' said Doug Frith, an MP from the northern Ontario district of Sudbury.
Mr. Trudeau has promised to resign before the next election, but he has left the timing open. At the moment, he and his three sons are on holiday in the Mediterranean, so there has been no reaction to this midsummer political tempest.
Liberals worry that if Mr. Trudeau does not resign soon, it will be tough for a new leader to catch on with the public and catch up with the Progressive Conservatives.
The new Conservative leader, Brian Mulroney, leads the party for which 55 percent of Canadians would vote if an election were held now. The Liberals have 27 percent and the socialist New Democrats have about 16 percent support. Since Canada uses the British ''first past the post'' electoral system, 55 percent of the vote would produce a landslide. Mrs. Thatcher won her majority in Britain this year with less than 50 percent of the vote.
In the 282-member House of Commons, the Liberals have 147 seats, the Conservatives 101, and the New Democrats 31. There is 1 independent MP and there are 2 vacancies. If an election were held now, the Conservatives would have as many as 200 members, the New Democrats probably fewer than 10, and the Liberals the rest.
The Liberal Party has always presented a united front, unlike the Conservatives, who constantly squabble in public. The call for Mr. Trudeau to resign breaks that tradition.
All concerned admit that if Mr. Trudeau wants to stay, it is going to be tough to push him out.
For one thing, the Liberal Party constitution does not appear to have a clause under which a leader can be forced to resign. And an observer in Ottawa noted there are few Liberals who can match Mr. Trudeau in ability or appearance. He may be low in the polls now, but he has been in power longer than any other Western leader. When Pierre Trudeau first became prime minister, Lyndon Johnson and Charles de Gaulle were still in power.
The favorite to replace Mr. Trudeau appears to be John Turner, a Toronto lawyer who resigned from the Liberal Cabinet as finance minister in 1975. Although Mr. Turner would certainly love to be prime minister, he may not be eager to lead a party that appears to be destined to spend the rest of the 1980s as Her Majesty's loyal opposition.