In 2014, the Independence Institute — a Colorado think tank — wanted to run a radio advertisement supporting the Justice Safety Valve Act, a bill granting federal judges greater discretion in sentencing nonviolent offenders. The text of the ad asked listeners to “call Senators Michael Bennet and Mark Udall” — Colorado’s two senators at the time — and tell them to support the bill. But under the Bipartisan Campaign Reform Act of 2002 (BCRA, better known as McCain-Feingold), any organization that spends at least $10,000 on “electioneering communications” in one year is required to make several public disclosures, including “the names and addresses of all contributors who contributed an aggregate amount of $1,000 or more” toward the advertisement. Further, an “electioneering communication” is defined as any broadcast that “refers to a clearly identified candidate for Federal office” within 60 days of a general election. Since Udall was running for reelection that year, the ad would have qualified even though it had nothing to do with Udall’s campaign. The Independence Institute challenged the rule as an unconstitutional burden on its First Amendment right to speak on issues of public concern. After losing before a three-judge district court, the Institute has now appealed directly to the Supreme Court. Cato, joining the Institute for Justice, has filed a brief urging the Court to grant the case a full hearing on the merits. We make two broad points. First BCRA’s disclosure provision is undeniably content-based, which should subject it to strict scrutiny under the First Amendment (meaning the government needs to provide a compelling justification). The law applies only if a speaker chooses to make reference to a candidate for office, so the law expressly draws distinctions based on the expressive content of speech. Second, mandatory-disclosure laws chill speech by forcing people to surrender their “privacy interest in keeping personal facts away from the public eye,” as the Supreme Court put it in U.S. Department of Justice v. Reporters Committee for Freedom of Press (1989). In the context of reviewing disclosures made under the Freedom of Information Act, the Court has recognized that “embarrassment in ... social and community relationships” is among the consequences of disclosure that “must be given great weight.” U.S. Department of State v. Ray (1991). Exactly the same analysis holds true for donors to advocacy organizations. For many people — without tenure, without salary protection, and without security details — government-mandated disclosure of their political leanings and personal data is a real barrier to political participation. Forcing people to divulge their personal information threatens to expose them to reprisals, and this deterrent effect is pervasive precisely because it is impossible to predict whether your viewpoint will trigger retaliation. BCRA’s disclosure rule is content-based, intrudes on speech and association, and has not been shown to serve a legitimate governmental interest. Because enforcement of the rule raises a substantial question under the First Amendment, the Court should take the case and ultimately overturn the district court.