Organised labour unions in the U.S., and their history. At the beginning of the twentieth century, only about 3 percent of the country's labour force belonged to unions. Up to the 1930s, unions were actively suppressed by employers. Workers inclined towards organising were often fired and blacklisted, and sometimes even beaten up or locked out of the plant. The courts often ruled that union attempts to increase wages and influence working conditions through strikes and picketing were illegal.

But membership grew nonetheless, especially during the Great Depression of the 1930s. By the 1960s, over 30 percent of the labour force was unionized. Since then membership has declined, in part because of the decline of highly unionised industries such as railways and the clothing trade, and the increase in white-collar workers, who have less of a tendency to organise than blue-collar workers. By 1990, the percentage of the labour force that was unionised dropped to about 18 percent. In 2009, it was down to 12.3 percent. The political influence of the labour movement has declined accordingly.