In spite of substantial public controversy, very little reliable data exists concerning the frequency with which non-citizen immigrants participate in United States elections. Although such participation is a violation of election laws in most parts of the United States, enforcement depends principally on disclosure of citizenship status at the time of voter registration. This study examines participation rates by non-citizens using a nationally representative sample that includes non-citizen immigrants. We find that some non-citizens participate in U.S. elections, and that this participation has been large enough to change meaningful election outcomes including Electoral College votes, and Congressional elections. Non-citizen votes likely gave Senate Democrats the pivotal 60th vote needed to overcome filibusters in order to pass health care reform and other Obama administration priorities in the 111th Congress.