The first major piece of legislation that affected labor unions was the Sherman Antitrust Act of 1890. The law forbade any "restraint of commerce" across state lines, and courts ruled that union strikes and boycotts were covered by the law. This was ironic since the Sherman Act had been passed by liberal reformers hoping to curb the abuses of business cartels and monopolies, not to crack down on unions.
Hope this helps! Took me awhile to type!