Can protests cause political change, or are they merely symptoms of underlying shifts in policy preferences? This paper studies the effect of the Tea Party movement in the United States, which rose to prominence through a series of rallies across the country on April 15, Tax Day, 2009. To identify the causal effect of protests, we use an instrumental variables approach that exploits variation in rainfall on the day of the coordinated rallies. Weather on Tax Day robustly predicts rally attendance and the subsequent local strength of the movement as measured by donations, media coverage, social networking activity, and later events. We show that larger rallies cause an increase in turnout in favor of the Republicans in the 2010 Congressional elections, and increase the likelihood that incumbent Democratic representatives retire. Incumbent policymaking is affected as well: representatives respond to large protests in their district by voting more conservatively in Congress. Finally, the estimates imply significant multiplier effects: for every protester, Republican votes increase by seven to fourteen votes. Together our results show that protests can build political movements that ultimately affect policy, and they suggest that it is unlikely that these effects arise solely through the standard channel of private-information revelation.