Source: Bureau of Labor Statistics, U.S. Dept. of Labor
| Year | Labor force 1 (thousands) | Union members 2 (thousands) | Percentage of labor force | |
| 1930 | 29,424 | 3,401 | 11.6 | |
| 1935 | 27,053 | 3,584 | 13.2 | |
| 1940 | 32,376 | 8,717 | 26.9 | |
| 1945 | 40,394 | 14,322 | 35.5 | |
| 1950 | 45,222 | 14,267 | 31.5 | |
| 1955 | 50,675 | 16,802 | 33.2 | |
| 1960 | 54,234 | 17,049 | 31.4 | |
| 1965 | 60,815 | 17,299 | 28.4 | |
| 1970 | 70,920 | 19,381 | 27.3 | |
| 1975 | 76,945 | 19,611 | 25.5 | |
| 1980 | 90,564 | 19,843 | 21.9 | |
| 1985 | 94,521 | 16,996 | 18.0 | |
| 1986 | 96,903 | 16,975 | 17.5 | |
| 1987 | 99,303 | 16,913 | 17.0 | |
| 1988 | 101,407 | 17,002 | 16.8 | |
| 1989 | 103,480 | 16,960 | 16.4 | |
| 1990 | 103,905 | 16,740 | 16.1 | |
| 1991 | 102,786 | 16,568 | 16.1 | |
| 1992 | 103,688 | 16,390 | 15.8 | |
| 1993 | 105,067 | 16,598 | 15.8 | |
| 1994 | 107,989 | 16,748 | 15.5 | |
| 1995 | 110,038 | 16,360 | 14.9 | |
| 1996 | 111,960 | 16,269 | 14.5 | |
| 1997 | 114,533 | 16,110 | 14.1 | |