The oldest of America's baby boomers are sixty years old this year. Almost one-fourth of Americans alive today were born in a population explosion between nineteen forty-six and nineteen sixty-four. As they retire, they will leave a labor market very different from the one they entered.

In the middle of the twentieth century, one worker in three was a member of a labor union. Now it is one worker in eight.

American unions had their greatest influence in the fifties and sixties. In nineteen fifty-five, the American Federation of Labor joined with the Congress of Industrial Organizations. The first president of the A.F.L.-C.I.O, George Meany, was a political force until he stepped down in nineteen seventy-nine. 

Last July, the federation suffered a split that included the loss of the fastest growing union in the country. The Service Employee International Union has almost two million members. Its president, Andrew Stern, says unions today must organize workers at big international companies. 

He supports a new labor federation, Change to Win. The unions in Change to Win together claim six million members. The A.F.L.-C.I.O. now has about nine million.

Industrial changes have hurt some unions more than others. Automobile industry unions have traditionally been among the strongest. But many of those jobs have disappeared as General Motors and Ford shrink their North American operations.

G.M. faces a strike threat at a major parts supplier. Delphi is seeking to cancel union agreements and cut pay. The United Auto Workers voted last month to permit a strike. Delphi, formerly part of G.M., is under bankruptcy court protection from its creditors. 

As the economy has changed, major new employers are companies like Wal-Mart. Wal-Mart says unions are not needed in its stores. It says it does not need a "middle man"in its relationship with its employees.

And now unions are facing a television campaign that uses humor to present a serious message. A group has gathered what it calls "a wealth of information" about the political and criminal activities of the American labor movement. 

The Center for Union Facts says it is supported by foundations, businesses, union members and the general public. It does not name its supporters.