Prime Minister Margaret Thatcher's Conservative Party appears to be heading for a decisive victory in its bid for reelection on June 9, but Thatcher may still fall short of the "unusually large majority" she has said she is seeking.

Thatcher returned from her weekend trip to the Williamsburg summit this morning to resume campaigning. The political news that awaited her here was good. Every published poll shows the Tories with a consistently solid margin over the second place Labor Party and the Alliance of Liberals and Social Democrats.

But according to Robert Worcester of Market Opinion Research International, one of the country's leading pollsters, the Conservatives are "piling up votes" mainly in their traditional strongholds and doing less well in areas where Labor has always been strongest. In an interview, Worcester estimated that Thatcher would have a majority of about 88 seats in the next Parliament, slightly less than the 100-seat edge that would represent a landslide.

The landslide issue emerged in the campaign because Francis Pym, Thatcher's foreign minister, said in a television program that so large a victory in past British elections had encouraged "extreme" policies by the victorious governments.

Thatcher rebuked Pym and said she would be able to do more in the "wider world if we have the authority of a big majority and a very, very large vote." She said an unusually large margin would enable Britain to give "a clear lead internationally" in support of the western alliance.

That attention at this stage of the campaign should be focused on predicting the size of the Conservative win reflects the serious problems facing Labor and the Alliance.

Labor has become embroiled in an internal dispute over the party's support for a policy of unilateral nuclear disarmament. Leading spokesmen, including deputy leader Denis Healey and former prime minister James Callaghan, have openly defied party leader Michael Foot's commitment to scrapping Britain's nuclear force within five years after taking power.

In fact, the disagreements on defense are not as great as they seem. The party is at least publicly pledged that a Labor victory would lead to an immediate reversal of plans to deploy U.S. medium-range nuclear missiles in Britain beginning later this year and cancellation of Britain's purchase of the U.S. Trident D5 submarine-launched missiles.

But the argument underscores the Labor campaign's biggest problem--a widespread feeling that, as The Economist put it this week, Foot "has been unsuited to be Labor's leader and he is even more unsuited to be prime minister."

The Conservatives are hitting extremely hard at the left-wing nature of Labor's platform. Full-page advertisements in all the country's major newspapers juxtapose 11 key points in Labor's program with identical proposals in the manifesto of Britain's small Communist Party. The headline is: "Like your manifesto, Comrade."

The Alliance, asserting that Labor is now a sure loser, is hoping to attract some of its support and thereby establish the centrist grouping as the country's main opposition to the Tories. In a weekend strategy session, the Alliance reportedly agreed to increase the public visibility of its most popular figure, Liberal leader David Steel.

In the closing days of the campaign, the Alliance hopes to benefit from a fall-off in Thatcher's poll ratings, which is considered likely based on the pattern in previous elections.

Thatcher's personal style continues to be one of the most discussed features of the campaign among politicians and in the British media. Her unchallenged supremacy as Conservative leader, her "headmistress" manner at party press conferences and her brusque treatment of moderates in the Cabinet like Pym have led many analysts to conclude that the election is increasingly a referendum on just how powerful the nation wants Thatcher to be.