Threat to Britain's two party system

The British political scene is more unstable than at any othertime in half a century. Breakup threatens the two-party pattern, under which rule has alternated between Conservatives and Labour since World War I.

It is all happening in a very calm British way. When the Queen drives to the Palace of Westminster early next month to open a new session of Parliament, everything will seem as traditionally and splendidly unruffled as it has since the days of Queen Victoria.

What is causing the current political ferment is: popular disillusionment with the two main parties - Margaret Thatcher's Conservatives and Michael Foot's Labour; and the emergence of a credible third-party alternative, the Social Democratic Party (SDP)-Liberal Alliance.

The question is whether the latter will grow to become the governing party or prove a passing phenomenon. At present it seems to be growing.

Both the Conservatives and Labour are in trouble. In recent polls, the Conservatives have run third behind the SDP-Liberal Alliance and Labour. Some headlines have described Mrs. Thatcher as the most unpopular Conservative prime minister since Neville Chamberlain in the 1930s. In the most recent by-election, Labour came in third behind the Alliance (which won its first contested seat since its formation) and Conservative candidates.

There are three reasons the two main parties are losing popular support.

1. Neither of them has devised an effective solution to Britain's chronic postwar economic problems.

2. Each has shifted away from the center in the search for alternative solutions - the Conservatives to the right with Mrs. Thatcher's monetarist policies, and Labour to the left, with leftist Tony Benn poised to capture the party leadership or impose his thinking on it.

3. Both the Conservatives and Labour, in their doctrinal zeal, have overlooked the fact that the great majority of Britons are for compassion, fair play, and compromise, and against dogma, confrontation, and intolerance.

These in turn are the same three reasons Britons are turning to the SDP-Liberal Alliance.

The Alliance is a coalition of David Steel's small Liberal Party (which has 12 seats in Parliament) and of a growing number of former Labour Party members which constitutes the SDP. The latter has 21 parliamentary seats of which 20 are defectors from the Labour Party and one a defector from the Conservative Party. But the SDP's two best-known members, Shirley Williams and Roy Jenkins, both former Cabinet ministers, have yet to win reelection to Parliament.

Mr. Jenkins ran in a by-election in a ''safe'' Labour constituency in Warrington this summer and came within a whisker of defeating the Labour candidate.

Mrs. Williams is following his example by running in a by-election later this year in Crosby, also in the north. But Crosby has hitherto been a ''safe'' Conservative seat. In the last general election, the Conservative candidate had a majority of more than 19,000 votes.

The date of the coming by-election has yet to be fixed. But a public opinion poll published in the London Sunday Times of Oct. 25 showed that if it were held now, Mrs. Williams would be the winner (on the SDP-Liberal ticket) with 40 percent of the votes. The Conservative candidate would get 34 percent and the Labour candidate 25 percent.

The by-election the history books may record as marking the breakthrough for the SDP-Liberal Alliance and the beginning of the end of the Conservative and Labour two-party system was held last week in the London dormitory suburb of Croydon.

There, William Pitt, running as the Alliance candidate, captured the seat from the Conservatives, with 41 percent.

The significance of Mr. Pitt's victory lies in two things.

First, in earlier elections in the same constituency, running as a Liberal, he had proven a chronic loser at the bottom of the poll.

Second, that figure of 41 percent tallies with the 40 percent which British public opinion polls show as the degree of support which the SDP-Liberal Alliance now has nationally. (Mrs. Williams's share of the Crosby public opinion poll was 40 percent.)

British political analysts say that if a lackluster, albeit worthy candidate like Mr. Pitt can match the national rating for the Alliance and win at Croydon, the Alliance is not just a flash in the pan - and the national public opinion polls are getting it right.

What that will mean at the next general election, due in another two years, remains to be seen.