One of President Biden’s campaign promises, passing the Protecting the Right to Organize (PRO) Act, would remove the “secondary boycott” prohibition from the National Labor Relations Act, a provision which prevents unions from pressuring employers’ customers and associates in order to bargain with those employers effectively. This long-standing prohibition prevents unions and their workers from engaging in what is otherwise considered protected speech under the First Amendment, including picketing in public places. Some labor historians and commentators view the 1947 Taft-Hartley amendments, which codified the secondary boycott prohibition, as a reversal of liberal, New Deal policies. This Note shows, in fact, that both state and federal courts were deeply suspicious of the secondary boycott throughout the 1930s and 1940s. Even as state legislatures seemingly liberalized the law of labor protest in the early 1930s, state courts soon nullified these anti-injunction statutes through the application of common law tort principles. Likewise, the First Amendment right to picket declared by the Supreme Court in 1940’s Thornhill v. Alabama was quickly rolled back in the following terms when cases involving secondary picketing arrived at the Court. The history of the secondary boycott is not simply a cyclical one of repression, liberalization, and repression’s return. Labor advocates should approach reforms with a careful eye to prevent merely defederalizing the law of secondary boycotts by repealing the NLRA prohibition and leaving its regulation to the states, for even the most progressive jurisdictions in the New Deal era were hesitant to recognize secondary activity as a legitimate form of protest, and the Supreme Court’s First Amendment cases on labor protest leave little recourse for a legal challenge.