While some of the bombings were certainly related to Labor, those motivated by political beliefs seem to have begun in the US in 1886. There was a very large bombing in Los Angeles in 1910. The Senate in Washington was bombed in 1915. There was a series of bombings in 1919, and a very large one in New York in 1920. The string of NY bombings I referred to actually took place between 1940 and 1956. Twenty-two bombs detonated, out of at least thirty-three placed in the string. Wikipedia has some links that you can use as a point of entry for further verification.