London — Britain's fledgling Social Democratic Party has administered a severe jolt to the ruling Conservatives and to the opposition Labour Party, and appears set to give them both an even sharper shock in the fall.
It is a sign that a major reshaping of British party politics is in the making, with moderation an emerging theme.
At a parliamentary by-election in the supposed Labour stronghold of Warrington in northern England, the SDP candidate, Roy Jenkins, ran his Labour opponent a close second.
Mr. Jenkins, a former Labour Party deputy leader who until seven months ago was president of the European Commission in Brussels, humiliated the Tory candidate, who did not win enough votes to regain his election deposit.
Now the Social Democrats, who fought Warrington in league with the Liberal Party, are turning their eyes to another by-election at Croydon, near London.
Their most likely candidate is the former Labour government minister, Shirley Williams. She is rated as a favorite to win if she becomes the joint SDP-Liberal standardbearer.
Meanwhile, Conservative and Labour Party planners are bewildered that a party barely six months old is proving able to win huge numbers of votes from both halves of the British political spectrum.
Warrington was a hard seat for Mr. Jenkins to tackle. A rotound intellectual with a prominent lisp, he has an image at once London- and Europe-oriented. His Labour opponent said he was a carpetbagger from Brussels and taunted him with his passionate commitment to British membership in the European Community -- not a popular line to take in northern England.
Against Mr. Jenkins, Labour put up Doug Hoyle, a former MP who is a member of his party's national executive committee with a record of support for left-winger Tony Benn. Mr. Hoyle tried hard to avoid identification with Mr. Benn's policies.
Carrying the Tory flag into battle was a London bus driver, Stanley Sorrell. He chose law and order and tough policiing as his main theme.
But even in a period of widespread rioting his line of approach was not appealing enough to puncture Mr. Jenkin's basic argument that Britain needs a party of the center dedicated to moderation and reconciliation.
When the result was announced, Mr. Hoyle got 48 percent of the vote, Mr. Jenkins 42 percent and Mr. Sorrell 7 percent. A jubilant Mr. Jenkins declared: "This is the first parliamentary election I have lost in 35 years, but it is by far the greatest victory in which I have ever participated."
The Croydon by-election seems a good prospect for the SDP because the Tories appear vulnerable and Labour is thought unlikely to do well anyway.
But it may be some time before Mrs. Williams is selected as candidate. Under a Liberal Party-SDP agreement, it would normally be the turn of the Liberals to contest Croydon, and their party already has a prospective candidate.
William Pitt bears a famous name, but the Liberal leader, David Steel, wants him to step down in favor of Mrs. Williams. It seems certain that Mrs. Williams , who is keen to run, will eventually be asked to fight the seat.
At that point the threat to the Conservatives and Labour will become acute. Mrs. Williams is an excellent campaigner and a highly popular figure nationally.
Political analysts say that if the swing Mr. Jenkins achieved in Warrington happened nationwide, the SDP and Liberals would form the next government. That is unlikely.
But if Mrs. Williams can seize Croydon, the SDP and Liberals will argue that they constitute a credible third force offering voters an alternativ e to extremism of all stripes.