FOR MANY YEARS the union share of the work force has been declining, but unions still have been the pacesetters in benefits and pay. Now that position has been affected as well. Union wages and benefits are still higher than non- union. But in each of the last two years the compensation of non-union workers has risen faster than the compensation of their unionized counterparts. Last year non-union compensation in the private sector increased 4.6 percent, union compensation, 2.6 percent, and union compensation in goods- producing industries, where unions traditionally have been strongest, only 1.9 percent.

Non-union workers have occasionally gained ground on unionized before, but never for such a sustained period in the years the Labor Department has been collecting such statistics. And the evidence suggests that the trend may continue for a while. First- year wage increases in major labor contracts signed last year were only 2.3 percent, and annual wage increases over the life of the contracts, not counting cost-of-living provisions, only 2.7 percent.

The Labor Department statistics also show a shift in the mix of benefits and pay. For many years benefits have tended to be the fastest-rising component of compensation costs. Employers in recent years have responded by trying to brake these -- health insurance and retirement costs especially. Last year benefit costs rose less than wages.

The faltering among unions is part of a pattern that has become familiar. Unionized industries have been among the hardest hit in recent years by competition from abroad. The union strongholds of autos and steel come first to mind. Among other factors, their foreign competitors have had the advantage of lower labor costs. Unions have also been victims of deregulation, particularly in trucking, airlines and elsewhere in the transportation field. And there has been a steady shift of strength in the economy from goods-producing industries, where unions have flourished over the years, to service- providing industries, where they have not.

The yielding that unions have had to do at the bargaining table has held down labor costs generally. Employment costs in the private sector rose only 4.3 percent last year, as against 5.2 percent the year before. That has both hurt and helped the economy. It has had a depressing effect on family incomes, to some extent on upward mobility and on purchasing power and the level of economic activity generally. At the same time, given that one man's income is another's costs, it has helped hold down inflation. That has been one of the fundamental trade-offs of recent economic policy.