The British Labor Party -- once described by Harold Wilson as "a broad church" -- has been so extended a coalition that it has embraced near-communists and establishment social democrats, pacifists and stout upholders of the NATO alliance, those who detest Britain's membership in the European Community and those for whom it is a commanding political purpose.

Now under the strain of constitutional changes and policy commitments, that extended coalition is breaking apart. Four ex-cabinet ministers -- Roy Jenkins, who was chancellor of the exchequer; David Owen, who was foreign secretary; William Rodgers, who was transport secretary; and I, education and science secretary -- backed by 10 other members of Parliament, have left the Labor Party and told our districts we cannot run again as Labor Party candidates.

In Britain's tough and disciplined party politics, dominated by political machines linked respectively to business and to the trade unions, such reckless bucking of party loyalties threatens a person's political prospects, career and even livelihood. Most of the MPs are people of high promise in their late 30s or 40s, who have held positions in government or as spokesmen in opposition, with a prestigious future ahead of them. All have put that future on the line.

Why? The answer goes back 10 years or more. In the late 1960s, the Labor right -- tough, autocratic and dominated by three powerful trade unions, lost control when the largest of those unions, the million-strong transport and general workers, elected as its general secretary a left-wing unilateralist, Frank Cousins, to succeed a little known right-winger. From then one, the party's 31-strong National Executive Committee changed its character. One by one, right-wing members were replaced by left-wing members, and the left mounted a campaign to control the party and to alter fundamentally its constitutional structure, which was based on a delicate balance of power between the parliamentary party of Labor MPs elected by the people and the party outside Parliament, whose half-million individual members and eight million affiliated trade union members elect the executive.

The left's first move was to introduce mandatory reselection of MPs -- that once in a Parliament's five-year life, each MP would have to go through a process of reslection by the general management committee of his local party, made up of 50 to 100 local activists. The process has to take place whether or not the management committee confirms its support for the MP.

Reselection is a necessary instrument to recall idle or corrupt MPs; but it is being used not to weed out the unsatisfactory, but to threaten those whose views on policy issues differ from the opinions of the the dominant left. wMPs who support British membership in NATO or in the European community, or incomes policy -- all of them policies supported in Labor's last election manifesto -- lay themselves open to attack and the threat of reselection by left-wing local management committees.

The left on the National Executive Committee next tried to wrest away any control over the manifesto -- the party's platform -- from the parliamentary committee with which at present that control is shared. The parliamentary committee, elected by Labor MPs, is bound to consider restraints of resources, budgets and legislative time in deciding how many of the party's sweeping, sometimes contradictory and often impractical proposals to adopt. The party in the country suffers from no such constraints. A manifesto drawn up by the party alone is a recipe for election promises that cannot be kept.

The third issue concerns the election of the party leader, who in the British political system becomes the prime minister if his or her party wins a majority in an election. Until this year, the leader of the Labor Party was elected by the Labor members of Parliament. At the special conference at Wembley on Jan. 24, delegates decided instead to set up an electoral college, 30 percent of whose members would be chosen by Labor members of Parliament, 30 percent by delegates from local parties and 40 percent by delegates representing trade unions affiliated with the Labor Party. The representatives of the constituencies would be mandated by local meetings, not be a secret individual vote of party members. The trade unions would cast a block vote based on the size of their affiliated membership; in some unions, that vote would certainly be cast on the decision of one or perhaps half-a-dozen senior officials, some of them appointed for life.

For the breakaway group, that was the final straw. Until a decade ago, Conservative leaders "emerged" from conclaves of a few landowners and grandees, known as "the magic circle." Now they are elected by Conservative MPs. For Labor to abandon the representative method in favor of "a magic circle" of trade union bosses and dubiously instructed local delegates was to us unacceptable; given that it might be the method of choosing a future prime minister, it was outrageous.

Together, these constitutional changes add up to a change in the character of the Labor Party. Increasingly, it is drifting toward a form of party control over elected representatives of the people, and away from representative democracy. It is this change in character that is more and more difficult to reverse.

In parallel with these constitutional developments, there have been changes in Labor's policy. Labor is now committed by a resolution passed at its October 1980 annual conference to the withdrawal of Britain from the European Community. There was no reference in that resolution to a referendum, though in 1975 pro- and anti-EEC cabinet members in the then Labor government agreed on a referendum to decide whether Britain should stay in the community as a way of uniting the party. Labor is every which way on defense -- for unilateral nuclear disarmament, for staying in NATO, for leaving its command structure, for removing U.S. nuclear bases from these islands. The best of all worlds cannot be the basis of a serious defense policy. Labor's domestic policy decisions add up to a further extension of state bureaucracy and to a whole set of conflicting and expensive priorities. No one wanted to say no, there being no political mileage in making unattractive choices.

Our group has fought for years within the Labor Party. Four years ago we established a body to fight for sensible social democratic policies. We invited all our colleagues on the party's moderate wing -- Denis Healey, Roy Hattersley, Merlyn Rees, Eric Varley, Roy Mason and many others -- to take part. Some to their credit, did. Most gave only nominal support. The core of the organization consisted of those who have now left, having sadly concluded that the fight was unwinnable.

Yet, paradoxically, it may be won just because we've left. Our leaving has concentrated the minds of the remaining moderates wonderfully, as Dr. Johnson once said the prospect of hanging does.

One hundred fifty Labor MPs have declared their readiness to fight the far left. The moderate trade unions have girded up their loins to reverse the decision made at Wembley's special conference in January. Everywhere sounds are heard of preparation for battle. On their side, the Conservatives are moving back toward the middle ground of politics. There have been significant speeches by Francis Pym, leader of the House of Commons, and Lord Thorneycroft, chairman of the party, asserting the need for moderation. The emergence of social democrats has unquestionably rattled the Conservative Party.

Meanwhile, the tide of the social democrats flows on. The most recent opinion polls give an electoral alliance between Britain's small traditional center party, the Liberals, and the social democrats, whose party does not yet even exist, over 40 percent of the vote, compared with just over a quarter each for Labor and Conservative. No results like this have been seen before in Britain in the whole history of opinion polls. Letters arrive by the thousands, many of them containing unsolicited contributions of money.

It is evident that some nerve has been touched among the British people, some yearning for a new beginning. The Conservaties have deeply disillusioned many oftheir supporters as the toll of lost jobs and bankrupt companies mounts. Margaret Thatcher, for all her courage, is seen as cold, rigid and divisive. Labor has headed too far toward the wilder shores of left-wing politics to appeal to Britain's solidly democratic electors. Never has the time been so ripe for a democratic left-of-center new party.