Britain's embattled trade union movement hangs in the balance

This should be the time when all good members of Britain's vast and once powerful trade union movement come to the rescue of the embattled organization. Down in membership, down in revenues, down in public esteem, Britain's trade union movement hangs in the balance.

Solidarity, the glue that held the trade union movement together through thick and thin during the depression years and that has been a powerful force in daunting successive British governments, is cracking badly. And largely from within.

Steelworkers, one of the three props in the triple alliance of steelworkers, coal miners, and railwaymen, are irate that militant coal miners want to threaten their jobs by limiting coal deliveries to steel furnaces.

Many dockworkers upset that they have been called out on strike only six weeks after the last stoppage say they won't go along this time. They claim the campaign is politically inspired and has nothing to do with the future of their jobs.

Moderate trade unionists who only want to go down the path of legitimate industrial action are at odds with militants in the movement out to achieve their ends through political means.

All this makes for an explosive mix when the conflicting views meet under one roof at the Trades Union Congress's annual meeting in Brighton next Monday.

Such internal rifts mask the external threats to the organization.

Unemployment is at record high levels with more than 3 million people out of work. This is one compelling reason for the sharp slide in membership among the older trade unions with accompanying loss in political clout. The public mood is hostile over repeated industrial stoppages in the coal fields and the ports. And trade unionists of moderate or militant stripe overwhelmingly see Prime Minister Margaret Thatcher as stridently anti-union.

''I said in a speech the other day that I never thought I would see the day I would call Macmillan a left-winger!'' said Ron Todd, general secretary-elect of the Transport and General Workers Union (TGWU), in a recent interview. Mr. Todd was alluding to former Conservative Prime Minister Harold Macmillan.

Mrs. Thatcher's combative style, her undiminished enthusiasm for priva-tization, her determination to curb union activities and ban trade union membership at the Cheltenham listening post - a move in which even a broad section of the public felt she had overstepped the mark - convince trade unionists that the prime minister is out to emasculate them.

With a trade union-reforming government and their ranks denuded by about 2 million unemployed workers, trade unionists feel they are in a state of siege.

Logically, the annual meeting of the TUC, which brings together under one roof white-collar civil servants and blue-collar boilermakers, provides trade unionists with a highly publicized platform from which to oppose government policies and practices.

Yet ask Gavin Laird, general secretary of the Amalgamated Union of Engineering Workers, Britain's second-largest union, which of the following issues will dominate the TUC meeting - Cheltenham, unemployment, trade union legislation, or the miners' strike - and he replies tersely: ''The latter.''

TUC moderates are worried that militant action by picketing miners could prompt police intervention and deflect the meeting from government strategies, allowing Mrs. Thatcher to make political capital out of union violence, and threaten to tear the TUC apart.