British Social Democrats show staying power

The Social Democratic Party has consolidated its position as a palpable third force in British politics, and its leader for the future is almost certain to be Roy Jenkins.

By his handsome by-election win March 25 in the Scottish seat of Glasgow Hillhead, Jenkins has boosted Social Democratic Party (SDP) fortunes and helped to clinch his claims to the party's leadership.

For the SDP and Jenkins a success at Hillhead was crucial. If the seat had gone to either the Conservatives or the Labour Party, the new party's fortunes would have begun to slide.

Jenkins staked his political reputation on winning, and this added greatly to the drama of the contest. Had he failed, he would almost certainly have had to relinquish hopes of gaining the leadership of the party.

The moment his win was announced, other leading SDP politicians indicated they were prepared to serve under him as leader. Associates of Liberal Party leader David Steel confirmed that he would be prepared to accept Jenkins's leadership of the SDP-Liberal alliance.

If the alliance can hold together, it has a good chance of driving a wedge between Prime Minister Margaret Thatcher's Conservative Party and Michael Foot's disunited Labour Party.

Minutes after the Hillhead result was known, Jenkins, a plump, assured politician who has served as both home secretary and chancellor of the Exchequer , declared, ''I want to offer a new choice and a new hope to the people of Britain. If that involves offering myself for the prime ministership, obviously I will not say no.''

In the wake of its Hillhead success, the SDP is expected to speed up the process of choosing a national leader. So far the party has had a joint leadership, its so-called ''gang of four.'' It consists of Shirley Williams, William Rodgers, David Owen (at present their parliamentary leader), and Jenkins.

Officials of the SDP are saying Jenkins's claim to the overall leadership is now irresistible. With a by-election coming up soon at Beaconsfield, the Social Democrats want to notch up another success.

Tony Benn, focus of much left-wing Labour Party sentiment, claimed the manner of the SDP victory offered hope for Labour. In winning, Jenkins wooed away more Tory than Labour voters, he said, and in a general election Labour might even win by default.

This possibility worries Mrs. Thatcher's Tories, but another prospect is opened up by the performance of the SDP: a parliamentary coalition between the Tories and the SDP-Liberal alliance aimed at preventing Labour from forming a government.

All this is in the future. What is certain for the present is that Jenkins's return to the House of Commons after an absence of five years signals a fresh phase in one of the most distinguished parliamentary careers of recent times.

Observers note that Jenkins had the courage to quit politics, take on the job of Europe's top civil servant, return to Britain, and help construct the political vehicle for his reentry on the national scene.

The nerve and boldness required were considerable. Allied to this, in Hillhead and earlier in a by-election at Warrington, which he narrowly lost last year, Jenkins has established himself as an energetic and effective campaigner.

Operating as the alliance's national leader and having a secure seat in the House of Commons would add up to a formidable combination of strengths. His commitment to European unity will strengthen the pro-Europe lobby in Britain.

Even Jenkins's opponents concede he is a brilliant man and has an enviable record in high government.

With Labour's Michael Foot in deep difficulty, Roy Jenkins could quickly establish himself as the most significant politician at Westminster after Margaret Thatcher.