THE BRITISH miners' strike is turning into the kind of event that leaves deep and enduring marks on a country's political life. Just as the miners' strikes in the 1920s foreshadowed the rise of the Labor Party as one of the country's two great political forces, the present strike looks very much like another sign of the party's decline.

For more than 60 years the unions have been a crucial element in Britain's political structure. Through their party, they gave parliamentary representation and expression to the half of Britain's people who describe themselves as the working classes. Now, under increasingly radical leadership, the miners' union is deliberately operating outside the law. It is running the strike with widespread violence, jeering at court orders and fighting the police. Last week the party's annual conference denounced the police -- over protests by the police union and weak remonstrances of the party leader -- and showed that the radical spirits were very much in charge.

The strike is not over wages but over the way the country is to be run. Most of Britain's nationalized mines are grossly uneconomical and exist only through huge subsidies. The government decided to close some of the least efficient mines. The miners are striking for their jobs -- that is, for the principle that inefficiency and economic losses should not affect traditional employment in public industries. That, of course, is a direct challenge to Mrs. Thatcher's central purpose, reforming the British economy and strengthening its ability to compete. The strike has become an explicit attempt to destroy the Thatcher government.

One point for the strikers: industrial change is most painful when new jobs are scarce. Unemployment is currently 13.6 percent. But if the miners deserve sympathy, the union does not. Its president, Arthur Scargill, has from the beginning adamantly refused to allow a strike vote, in violation of the union's rules. Instead he is using massive picketing to try to intimidate the substantial minority of his members who, in the absence of a vote, continue to work. The other unions are giving him support that ranges from lukewarm to nil. But his methods seem to be having a certain influence within the Labor Party, where the red-hots in some constituencies are preparing a purge of its more moderate members of Parliament.

The strike has reached a point at which compromise is difficult. Any concessions to the miners would be a great victory for the union and a dangerous defeat for the government. Meanwhile the rudderless Labor Party entangles itself further in internal quarrels and the cranky causes of the far left. That leaves Mrs. Thatcher and her Conservatives without the steadying influence of the strong, coherent opposition that, in a time of rising tempers, they will need.