London — In one of the potentially deepest shifts in Britain for 50 years, the face of politics in this bastion of democracy and of the Western alliance is assuming an entirely new look.
Now begun is a sustained battle for the center-right, the center, and the center-left of the British political spectrum, where general elections are won and lost and where governments rise and fall.
The battle, however, is not between Conservatives and Labour, but between Conservatives and the formidable new challenge of the alliance of the Social Democratic Party (SDP) and the Liberals.
On the one hand is a revived, newly popular, and vigorous prime minister. Margaret Thatcher has swung into a new campaign based on the victory in the Falklands war, declaring (as she did forcefully in a speech July 3) that the war ''proved ourselves to ourselves.''
She attacks striking rail, health, and other unionists with rhetoric: ''All over Britain,'' she said, ''men and women are asking, 'why can't we achieve in peace what we can do so well in war?' ''
Facing her in showdown combat is now one of the best-known faces and voices in British politics in the last two decades, the new leader of the Social Democratic Party itself: Roy Jenkins.
Mr. Jenkins, a former Labour chancellor of the exchequer and home (internal affairs) secretary, and former president of the European Commission in Brussels, has done the most to create the biggest realignment of British politics since Labour's Ramsay MacDonald agreed to lead a national coalition in 1931 and opened the door to nine years of effective Conservative control of government.
It was his opposition both to Conservative right-wing theories and to the increasingly vocal and powerful Trotskyite far left-wing of the Labour Party that led to the founding of the SDP early last year.
Now he holds the limelight among all those opposed to the Thatcher government.
By electing him as SDP leader July 2 over the younger, more abrasive, more radical-appearing David Owen (a former Labour foreign secretary), SDP members decided on a broad party appeal to the center ground of the electorate, rather than a narrower appeal to disaffected Labour voters.
Mr. Jenkins appeals mainly to the left of the Conservative Party, but insists he can also pull in the right of Labour as well.
The immediate significance of his election, in which he won almost 55 percent of 47,000 votes cast, is that Mr. Jenkins will almost certainly become leader of the still-new Alliance between the SDP and the Liberal Party.
Liberal leader David Steel is widely seen in Westminster as a capable and energetic party leader, but he is Dr. Owen's age, and his party has been so small for so long that he has had no chance to acquire the kind of experience in office that Mr. Jenkins has had.
So it will be the upper-middle-class Mr. Jenkins, and his rich and plummy accent which pronounces ''r'' as a languid ''w,'' that will lead the Alliance into the next election about 18 months from now.
One of his many obstacles is that, in the past, the British electoral system of ''first past the post'' voting has effectively kept smaller parties from power.
But the Jenkins SDP, and the Liberals, stake their hopes on appealing to British voters to ''break the mold'' - to vote for the Alliance so that it can either take power outright and bring in proportional representation, or demand the new system as the price for supporting one of the other parties in a hung Parliament.
Mrs. Thatcher herself told friends before the SDP poll that Mr. Jenkins would be a tougher opponent than Dr. Owen.
At Christmas last year the SDP were scoring above 40 percent in the opinion polls. This dropped to below 30 percent while the Conservatives dominated the headlines during the Falklands war. But Mr. Jenkins and his backers look for a revival after the Conservative Party annual conference this autumn.
Had Dr. Owen won the SDP election, the party itself would have altered character somewhat, and he would have found Mr. Steel strongly contesting Alliance leadership.
Mr. Jenkins stresses the importance of welding the Alliance into an alternative government. Dr. Owen, who polled strongly after an expert parliamentary performance during the Falklands war, sees the electoral system defeating the Alliance. He believes the SDP should try to supplant Labour as the main opposition to the Conservative Party in a two-party system. In a hung Parliament, the SDP should be ready to join unhappy Labour members in government , if necessary under a Labour leader.
Conservative Party tactics now are to brand the SDP as more socialist than democratic. The SDP is fighting back by defining itself as pro-Europe, pro-NATO, pro-pragmatism and flexibility at home, but against the US Trident nuclear deterrent. The SDP would retain US cruise missiles in Britain.