OSLO, Norway (AP) _ Discontent over the economy and the environment has caused many Norwegians to shift party loyalties, signaling danger for socialist Prime Minister Gro Harlem Brundtland in Monday's national elections.

''There is tremendous migration of voters from party to party, the likes of which we have never seen in Norway,'' said Per Arne Hestetun of the polling company Norsk Gallup A-S.

In an August poll of 7,038 people by Gallup, voters listed as their main concerns the pollution of Norway's unique fjords, a record unemployment rate of 5 percent, and care for the aged.

Many voters feel the prime minister, of the socialist Labor Party, has not worked hard enough to clean up Norway's environment. They also say they have yet to feel the benefits of her economic austerity program.

Ironically, Mrs. Brundtland is known abroad as a champion of conservation. She is chairman of the U.N. World Commission on Environment and Development, and foreign economists have praised her for turning around an economy sinking deeply into debt.

About 3.2 million Norwegians in this Scandinavian nation of 4.2 million are eligible to vote Monday for the 165-seat parliament or Storting, which has been expanded from 157 seats.

The elections come after a weekend suspension of campaigning to mourn 55 people, mostly Norwegians, who died Friday in a plane crash off Denmark.

Analysts predict no clear majority will emerge in Monday's voting. A large block of undecided voters, up to 20 percent, could swing the outcome.

A poll published Saturday in Oslo's Aftenposten newspaper predicted that Mrs. Brundtland's Labor Party will sink from 40.5 percent of the vote in the 1985 election to 33.7 percent. Her party has governed with a parliamentary minority of 71 seats and issue-by-issue support from other smaller parties.

The poll also said the Conservatives, the leading opposition party, will drop from the 30.4 percent of the vote they garnered in 1985 to 20 percent. It gave the right-wing Party of Progress 16 percent and the rest of the votes to a variety of small parties.

Based on the poll, Aftenposten predicted 86 non-socialist seats and 79 socialist seats in the new parliament.

The telephone poll of 1,215 voters was conducted Sept. 4-7 by the Bergen survey institute Opinion A-S for Aftenposten. The margin of error was 2.2 to 2.8 percent.

Pollster Hestetun said he believed 'Mrs. Brundtland will resign, maybe not on the eve of the election but shortly afterwards and that a non-socialist coalition will take over.''

Mrs. Brundtland appeared to be recovering some lost support Sunday, analysts said. Her non-socialist opponents retained an apparent majority, they said, but it was possible deep rifts would prevent them from forming a working alliance.

Mrs. Brundtland, 50, a doctor and U.S-trained specialist in public health, became prime minister of a minority government for the second time when a Conservative-led coalition resigned in 1986.

She has said a poor showing in Monday's vote will not automatically force her to step down, since an alternative to a Labor government remains unclear.

Conservative Party leader Jan P. Syse, 58, is the most likely alternative to Mrs. Brundtland. His party is allied with three smaller non-socialist parties.

Syse has said Labor is ducking responsibility by blaming his party, and that ''socialism is bankrupt.'' But his party and its allies have failed to find reach final accord on a candidate for prime minister or a common platform.

Christian Democratic chairman Kjell Magne Bondevik, a junior member of the non-socialist alliance, has put himself forward as a compromise choice with some public support.

The wild card in Monday's election is Carl I. Hagen of the right-wing Party of Progress, who is challenging the underpinnings of the welfare state by urging sharp tax cuts and privatization of state-owned industries and services.

Hagen has been shunned by the other parties for a decade. Distaste for him could unify the other non-socialist parties enough to overcome their differences and exclude him from government, analysts said.

Mrs. Brundtland has been accused of ignoring the goals of a government commission that drafted a 1987 environmental plan called ''Our Common Future.'' She has rejected the criticism as unfair politicking by her rivals.

The international Organization of Economic Development and Cooperation has credited Mrs. Brundtland for the austerity diet she imposed on the economy, which had grown fat on North Sea oil wealth and then crashed with a price collapse in 1986.

Despite her political leanings, Mrs. Brundtland imposed wage restrictions and other fiscal restraint that halved inflation in two years to under 5 percent and turned the foreign debt into a surplus.

However, unemployment and bankruptcies have hit record levels, giving a hollow ring to Labor's motto of ''Employment for All.''