In the context of labor law, the origin of the relationship between free speech and peaceful picketing is the case of Thornhill v. Alabama. The speech aspects of picketing and the necessarily accompanying First Amendment protections were stressed by the Supreme Court in holding unconstitutional a state statute which constituted a broad ban on all picketing. The Court expressly limited the holding by recognizing that a narrowly worded statute, such as one merely interdicting picketing en masse or picketing portending imminent danger may be valid. An additional limitation of the decision is the fact that it was reached under a "balancing of interests" test; consequently, the holding is impliedly qualified.
Corso, Joseph J.
"The Protection Accorded Picketing by the First Amendment,"
Akron Law Review: Vol. 2
, Article 3.
Available at: http://ideaexchange.uakron.edu/akronlawreview/vol2/iss2/3