Britain's Social Democrats have to choose between Jenkins and Owen

Britain's Social Democratic Party (SDP), born last year, is launching a leadership contest with two candidates in the field.

Roy Jenkins, former deputy Labour Party leader and once president of the European Commission in Brussels, is shaping up against David Owen, a former Labour foreign secretary, who enjoys support from younger elements in the SDP.

The contest between two members of the original ''gang of four'' -- a group who broke away from the Labour Party to create a new party of the center -- is not about policy. SDP rank and file are being asked to appraise the political personalities of the two men and to consider their respective ages.

Mr. Jenkins, though a much more experienced politician than Mr. Owen, is 20 years his senior. Supporters of Owen, including another member of the ''gang of four,'' Shirley Williams, believe a vote for him is a vote for the future of their party, which may have to battle hard if it is ever to achieve a majority in Parliament.

Three months ago it was thought that Jenkins, who returned to the House of Commons earlier this year after a by-election victory, would not even have to contest the SDP leadership. That was before the Falklands crisis.

Owen, an astute foreign affairs specialist, leads the SDP in the House of Commons, and immediately established himself as a highly informed and persuasive spokesman on the Falklands crisis.

Jenkins, by contrast, kept silent, renewing acquaintances at Westminster and attempting to prepare the ground for eventual assumption of the party leadership.

The two men, who remain the best of friends, have made it clear they will make no bitter personal attacks on each other. One SDP official said: ''The leadership battle will be conducted with due decorum, and the man who wins will enjoy total backing from the rank and file.''

Jenkins, a skilled debater with a strong humanitarian record and lengthy experience of economic and European affairs, is seen as the favorite in the race. But Owen is commanding much support among the 78,000 party members who will decide the outcome.

The Social Democrats have lost some momentum since earlier in the year, when their political star was in the ascendant and some commentators were predicting that if there were a general election now, they and their Liberal Party allies would be able to form a government.

But the Falklands crisis has tended to sap support for the SDP. At a by-election last week at which a former Labour Party member of Parliament tried to retain his seat as a Social Democrat, the Conservative Party candidate romped home.

The moral drawn by commentators is that the Falklands crisis, provoking strong patriotic emotions, made it difficult for the SDP to argue the need for a third party in the radical center of the political spectrum.

The SDP candidate had the satisfaction of seeing his Labour opponent come in third but, by wide consent within the SDP, there is need now for strong leadership.

If Owen fails to beat Jenkins at the post but piles up an impressive vote, his position in the SDP hierarchy will be strong, and he will be seen as a likely leader for the longer-term future.