Congress’s abuse of the subpoena power is not a new phenomenon. During the 1950s, Senator Joseph McCarthy infamously used this power to facilitate a series of closed-door hearings on “Un-American activities.” History has condemned McCarthy’s witch-hunt, but that episode did not mark the end of overzealous congressional committees. The latest example involves Backpage.com, an online intermediary providing a forum for classified ads, including rentals, real estate, jobs, and an "adult" section. In April 2015, the Senate Permanent Subcommittee on investigations (PSI) launched an investigation into Backpage and its CEO Carl Ferrer, citing a search for information on human trafficking in connection with the site’s adult section. After several months of preliminary investigation, the PSI issued what can only be described as a “blunderbuss” subpoena demanding that Backpage and Ferrer turnover information related to their advertisements and editorial decisions for the previous six years—a request covering millions of documents, some of which go to core editorial functions. The PSI eventually withdrew its subpoena, but issued a new one that it claimed was narrower in scope. In reality, the new demand merely reframed the original request using broader language. Ferrer challenged the new subpoena in a federal district court, arguing that its overbreadth—and the subjects it sought to uncover—violated his First Amendment rights and did not conform to the legislative functions that PSI must adhere to when using its investigatory powers. More specifically, the investigation was part of a concerted effort to target Backpage and punish the website’s protected—legal—content, such that the subpoena chilled his freedom of speech. The district court disagreed, finding that the PSI’s request for information outweighed Ferrer’s First Amendment rights. Ferrer has now appealed to the U.S. Court of Appeals for the D.C. Circuit. This is not the first time a government agency has used its power to target Backpage. Indeed, the PSI’s investigation builds on a spurious campaign by the Sherriff of Cook County, Illinois (Chicago) to punish Backpage by demanding that major credit card companies prohibit the use of their cards to buy ads on the site. In a case supported by Cato, DKT Liberty Project, and Reason Foundation as amici, the Seventh Circuit found that Sheriff Tom Dart’s actions constituted unlawful censorship. We have now filed an amicus brief with the D.C. Circuit in an effort once again to vindicate the constitutional rights of online intermediaries. At least Dart was acting in a law-enforcement capacity, with necessarily different powers than a congressional committee. When Congress exercises its investigatory powers, the investigation must be used solely in service of its legislative agenda. The PSI ignored this limitation, however, and flaunted the separation of powers when it acted as a law enforcement agency to target Backpage and Ferrer. Moreover, the congressional subpoena power is particularly circumscribed when the First Amendment rights of the recipient are at issue. The PSI wielded its authority improperly through excessive requests for information, all in the name of shutting down third-party content that they find offensive. PSI’s overreach is emblematic of a disturbing pattern of legislative committees’ and state attorneys generals’ brandishing their subpoena power as a weapon to attack individuals and entities that engage in disfavored speech or conduct. The D.C. Circuit should reverse the district court and make clear that Congress’s subpoena power is not a weapon to be used to chill speech or regulate conduct.