Trade unions have a tough time in the US up against the traditional American philosophy of free enterprise and individualism.

Membership of the unions has fallen steadily in the private sector, where it is now estimated at only 10 per cent. Organised labour is stronger in the public sector of federal and local government employees.

Under Irish-American Mr John Sweeney, the new leader of the AFL-CIO federation, a strong recruiting drive is under way in the private sector, such as in hotel workers and fruit-pickers in California.

Industrial law is loaded against workers in some areas. Under the Chapter Eleven bankruptcy law, a company can sack all workers, reorganise, and then hire a smaller, non-union workforce. American airlines made use of this power in the 1980s when TWA, Continental and US Air resorted to Chapter Eleven.

Workers in a company are entitled to join unions but only after a majority show they are in favour by an election. Employers often delay elections or make it clear to workers that they do not want a union. Immigrant workers on minimum wages are often afraid to provoke employer displeasure.

Unions got full legal recognition in the Great Depression era in 1937 with the Wagner Act under Roosevelt's New Deal. But the Taft-Hartley Act in 1947 weakened the power of unions by outlawing secondary industrial action and enforcing a cooling-off period. States were allowed to legislate against the closed shop.

The teachers' unions grew between 1960 and 1975 under Democratic administrations and Congresses. The membership of unions in the manufacturing and mining sectors is still declining, while workers in the growing service and hi-tech industries are more difficult to organise.

Since the 1970s, the real incomes of US workers have actually declined taking inflation into account, while those of top executives have soared. The unions have been more concerned with protecting job security of their members than seeking higher wages and better health insurance.

Under President Ronald Reagan the unions suffered further with a conservative backlash. Mr Reagan won widespread acclaim for crushing the air traffic controllers strike and abolishing its PATCO union which had actually supported Reagan in the election. President Clinton rescinded the ban on rehiring the sacked controllers.

The Teamsters union won a bitter strike last summer against the UPS freight company which was seen to be exploiting temporary workers. However, soon afterwards the Teamsters' boss, Mr Ron Carey, had his leadership win over Mr James Hoffa jnr declared void because of election fraud.

The AFL-CIO broke new ground in the 1996 Congressional elections by targeting vulnerable Republicans and running TV spots against them. The tactic had mixed success but failed to win control of the House of Representatives for the Democrats.

The unions showed their influence over the Democrats at the end of last year when they lobbied successfully against President Clinton's request to Congress for "fast track" authority to allow him to negotiate new trade agreements.