The Plumbers and Steamfitters Local Union 100 (Local 100) (defendant) represented plumbing and mechanical workers under a multi-employer bargaining agreement with an association of 75 mechanical contractors. The agreement included a “most-favored-nation” clause, meaning if the union entered a more favorable contract with another employer, it would extend the same terms to the association. Connell Construction Co. (plaintiff) subcontracted plumbing and mechanical work to union and nonunion subcontractors based on competitive bidding. Other trade unions represented Connell’s employees, and Local 100 had never tried to represent them, but wanted Connell to use only Local 100 members for onsite mechanical work. When Connell refused, Local 100 stationed one picket at a Connell worksite. About 150 workers walked off, halting construction. Connell signed the agreement under protest and sued, claiming it violated antitrust laws. Meanwhile, Local 100 obtained identical agreements from five other contractors and picketed those who resisted. The court found the agreement exempt from antitrust laws under the construction-industry proviso of the National Labor Relations Act (NLRA). The Fifth Circuit affirmed, reasoning that Local 100 had a legitimate union interest in organizing nonunion subcontractors. Connell appealed, and the Supreme Court granted review.