Using panel data and matching techniques, we exploit a rare change in communication flows—the endorsement switch to the Labour Party by several prominent British newspapers before the 1997 United Kingdom general election—to study the persuasive power of the news media. These unusual endorsement switches provide an opportunity to test for news media persuasion while avoiding methodological pitfalls that have plagued previous studies. By comparing readers of newspapers that switched endorsements to similar individuals who did not read these newspapers, we estimate that these papers persuaded a considerable share of their readers to vote for Labour. Depending on the statistical approach, the point estimates vary from about 10% to as high as 25% of readers. These findings provide rare evidence that the news media exert a powerful influence on mass political behavior.