WASHINGTON—As Congress returns from its Easter break, lawmakers are preparing to navigate the renewal of a controversial surveillance law. Section 702 of the Foreign Intelligence Surveillance Act (FISA), set to expire on April 20, enables the government to conduct warrantless surveillance of foreign targets outside the United States that are thought to pose a threat to the country. Its defenders, who now include President Donald Trump, say it is vital to protecting national security, particularly amidst the Iran War, and draw attention to 2024 reforms. Section 702 remains contentious because it can pick up incidental information on Americans without a warrant. That raises Fourth Amendment concerns among its critics....