WHAT HAPPENED in Britain last week may seen to be of only parochial significance. But it could transform politics there in a few years and, if it does so, will affect politics in all the western democracies, including here.

The numbers of politicans involved so far may seen to be pitiably small. Twelve members of Parliament resigned from the Labor Party, to form a new party which is to be known as the Social Democrats. What chance have they to make even a small dent in the ding-dong battle between the highly disciplined and centralized parties in Britain? They in fact have quite a chance.

The point about a party system which is apparently so cast iron as that in Britain is that a blow applied in the right place can split it apart. The history of the rise of the Labor Party itself tells a story. In 1914, it was a little band of 42 members of Parliament; the Liberals were ruling. In 1942, it formed a government; the LIBERALS WERE REDUCED TO 40 MPs. So quickly can the most powerful party be replaced by another.

The Labor Party has moved so fast and far to the left in recent years that it has all but taken itself out of the mainstream of British politics. Yet without a challenge from within its own leadership, it might still lumber to a victory against the Conservatives. It might still be carried of office on a wave of dissatisfaction with Mrs. Thatcher.

What the new Social Democrats have done -- four of their leaders were cabinet ministers in the last Labor government -- is insert themselves into the center of British politics. Especially if they form an electoral alliance with the Liberals, they could steal a march on both the major parties, and within one election become a formidable and hopeful new force. It is only a possibly; it is an exciting one.

Why it may matter elsewhere than in Britain is that, at a time when there seems to be only a marked shift to the right in the democracies throughout the world, this is one of the first signs of a new vitality on the left.

Consider what the Labor Party has done to itself. It has transformed itself in a few years into an isolationist party: for example, in its wish to dissociate Britian from the Common Market and even from NATO.

The Labor Party has transformed itself into an anachronistic socialist party: for example, in its wish to embark on a program of nationalization which bears no relationship to reality. What britian needs to do is loosen its mixed economy, and experiment more freely within it. it is suffering enough from the far-right ideology of Mrs. Thatcher. It does not need a far-left ideology as an alternative.

The Labor Party has, finally, abandoned the parliamentary tradition. The very heart of that tradition lies in the selection of a party's leaders by its elected MPs. The Labor Party has adopted a system which throws that selection to the party's militant workers and to the trade unions. Such a system would bind the MPs, who are supposed to represent the county, to a leadership with no parliamentary consituency.

It is againt all of these that the Social Democrats have revolted, and they are likely to be joined by others who are now more cowardly. The public opinion polls -- which show them gaining, in alliance with the Liberals, 40 percent of the popular vote -- are a little untrustworthy. That figure in part represents a normal midterm dissatisfaction with the major parties. Yet there is no doubt of the om dissatisfaction with the major parties. Yet there is no doubt of the opportunity.

The Social Democrats must immdiately be most concerned with organization and even political maneuver. But what will also be released is a great deal of new thinking among the moderate left. This will be the influence elsewhere. The Social Democratic parties in Western Europe need a morla and intellectual boost. Any Democrat in America concerned about restoring his party should watch the British development closely.

The social democratic or liberal tradition in our democracies in this century would have been a very factional affair if it had not been for the New Deal-Fair Deal politics of the United States and the Labor Party in Britain.

The restoration of parliamentary democracy in Western Europe had two inspirations: first, the superb assistance of the United States, itself the work of New Deal-Fair Deal men; and second, the preeminence of the Labor Party after 1945, and the leadership example which it provided. On the liberal left on both sides of the Atlantic, things have not been the same since that flowering a generation ago.

We are sometimes tempted to look only for great movements of opinion. Yet politics are affected by personalities. We will never know what would have happened if John F. Kennedy had lived, or if Lyndon B. Johnson had not been trapped in a war on the other side of the world. But let us concentrate on the Labor Party. One of the great "ifs" of modern history may be found in the deaths of two of its leaders.

Clement Attlee, prime minister of the great Labor government after the war, was succeeded by the best of his younger lieutenants, Hugh Gaitskell. There are some men for whom it is hard to find the right words of praise. Gaitskell was one of them. No politicain was a finer, more thoughtful, more courageous, more decent man than he.

This donnish man who went to one of the best schools in England, and to one of the best colleges at Oxford, had more genuine radicalism in his bones than many of his left-wing opponents on their tongues. I spent the whole of the Saturday after he was defeated by Harold Macmillan in 1959 at his home, Some think that politicians should not cry. Gaitskell wept in front of me, and not at his own disappointment.

He spoke of education; he spoke of health; he spoke of housing; he spoke of old age; he spoke of poverty. He spoke, above all, of justice.He had been defeated by the slogan: "You have never had it so good." Who was having it so good? He rose as he talked in his study, the tears moistening his cheeks: "Too many lives are still impoverished and stunted, and yet we are told that we have never had it so good."

Little more than three years later he was dead, struck down with appalling suddenness by a rare disease; one day we heard that he was ill, within a month we heard that he had died at 56.

This was a politician of principle. When he was defeated by his party's annual conference on the issue of unilateral nuclear disarmament, other moderate leaders of the party began within 10 minutes to shift their feet to the left. I talked to him after talking to some of them. No hint of sly maneuver or accommodation from him. His first words to me:

"There is only one thing that matters now: to keep the party behind the American alliance."

He went before the party conference which had voted him down. In the words which cannot be forgotten he was every inch the leader: "I will fight, fight, and fight again" -- his voice rang out over the catcalls -- "to bring this party back to sanity." A year later, at the next conference of the party, he did it. The party reversed its earlier decision. Such a man one could trust; such a man one would follow.

Those who have resigned from the Labor Party now are his disciples. It is no longer his party. It has gone too far to be brought back again to sanity. They are showing a measure of his mettle and his conviction. They are not to be discounted, nor their willingness to lead.

In his fight for the leadership of his party, his main foe had been Aneurin Bevan, the brilliant but erratic spokesman of the extreme left. But after the 1959 election, they made a pact. Bevan would support Gaitskell as the next Labor prime minister; Gaitskell promised to make Bevan his foreign secretary. It could have been a treaty of far-reaching significance. But it was not to be.

Bevan died in 1960 as abruptyly as Gaitskell two years later. The leadership of the Labor Party passed to Harold Wilson, a man who for his own advancement had emptied himself of all conviction. This politican of neither principle nor scruple probably contributed more than any other factor in his years as leader and prime minister to the now disheveled condition of both his country and his party.

The banner of social democracy has at least been picked up again by leaders with Gaitskell's intelligence and courage. British socialist thought, essentially non-Marxist, has always been fertile, even in recent years. Many followers of Gaitskell, among them Anthony Crosland, another leader who died tragically young, have continued the work of redefinition. It now in Britain has a focus.

The liberal Democrats in America ought to heed it. Where does social democracy find its inspiration? In its sense of justice, above all, the giving of justice to all. But where can it find its fire? It was Bevan, on the far left but never a Communist, who gave the bidding: "It must achieve passion in action in the pursuit of qualified judgments." There could not be a more testing exhortation.

To be passionate in action even though one's objectives are qualified. That is the temper which social democracy in Europe and liberalism here must recover. A start may have been made by 12 politicians on a small island. Other great adventures have set sail from there before.