The impressive election victory by former Labor Cabinet minister Shirley Williams to become the first elected Social Democrat in the House of Commons is widely interpreted by political commentators here as a turning point in British politics.

While last week's election does not by itself foreshadow a political realignment in Britain, it continues a trend now considered "a serious threat" by most Conservative and Labor politicians.

Only nine months ago, 12 members of Parliament, led by two former Cabinet members, broke away from the opposition Labor Party to form the Social Democratic Party in the House of Commons.

With Williams taking her seat on Tuesday, the Social Democrats' strength in the Commons, which had increased steadily with defections from Labor, has grown to 24.

Williams fills a vacant parliamentary seat from suburban Crosby, outside Liverpool, which had been one of the safest constituencies in the country for Prime Minister Margaret Thatcher's governing Conservatives.

The massive shift of both Conservative and Labor voters in Crosby to the Social Democrats, backed in an electoral alliance by the venerable centrist Liberal Party, closely matched the support the alliance has attracted in two previous parliamentary by-elections, dozens of elections to fill local government vacancies and national public opinion polls.

In all of these, an average of 45 percent of Britain's voters have showed consistent support for the Social Democratic-Liberal alliance, putting it well ahead of the Conservatives and Labor.

Political commentators are beginning to assume that the new alliance will at minimum force its way into a coalition government after the next national election in 1983 or 1984.

Both Thatcher and opposition Labor Party leader Michael Foot have fallen to record-low approval ratings in opinion polls showing clearly that the main voter appeal of the Social Democratic-Liberal alliance is as a centrist alternative to Thatcher's right-wing leadership and Labor's recent turn to the left.

British governments have been the victims of strong voter protests in mid-term elections in the past, but this is the first time since the 1920s that the protest has been against both parties.

The question remains whether this political protest will last until the next national election and solidify into a long-term power base for the Social Democratic-Liberal alliance.

Robert Worcester, of Market Opinion and Research International, said voter disenchantment with the Conservatives and Labor, and therefore support for the Social Democratic-Liberal alliance, could now be at its peak.

"The alliance could lose some voters when its policies and candidates are more closely examined by the voters," he said.

A recent survey of Social Democratic Party members revealed some conflicts between them and the vision of some of the party's founders. Two-thirds of questioned members saw it as a moderate party of the center.

This view conforms to that of the leading candidate for party leadership, former deputy Labor leader Roy Jenkins, but not to the left-of-center party of radical social change envisioned by two of its other founders and leaders, Williams and former foreign secretary David Owen.

But party members overwhelmingly support their leaders' current policies, including moderately increased government spending and business subsidies to reduce record unemployment, controls on wages enforced by tax penalties, continued membership in the European Community, and strong support for the NATO alliance while seeking worldwide nuclear arms reductions.

Social Democratic Party members divide almost evenly among previous Labor, Conservative and Liberal voters, but they are much younger and more middle class than the British population overall, the survey showed. A majority hold professional or managerial positions compared to just 16 percent in the British population overall.

Because the Liberals also have tended to be more middle class than the rest of the population, the alliance provides a political test of the belief of many social scientists here that more Britons identify today with middle-class aspirations than with occupational or labor union ties .

Unless either the Conservatives or Labor move back into the center, and there are no indications so far that either will, many politicians and analysts forecast that they will gain enough support only to win a third of the vote each in the next national election. This would likely leave no party with a majority in Parliament.

The Conservatives are believed by both political analysts and many senior Conservatives in Thatcher's government to be the most vulnerable to inroads by the new alliance among middle-class voters. An election defeat for them could lead in this view to Thatcher's replacement by a more moderate leader and a coalition with the alliance.