Do non-citizens vote in U.S. elections? Yes, and they make a difference, according to an academic paper by Jesse Richman et al.
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. …
Our exploration of non-citizen voting in the 2008 presidential election found that most non-citizens did not register or vote in 2008, but some did. The proportion of noncitizens who voted was less than fifteen percent, but significantly greater than zero. Similarly in 2010 we found that more than three percent of non-citizens reported voting.
Our results also suggest that photo-identification requirements are unlikely to be effective at preventing electoral participation by non-citizen immigrants: In 2008, more than two thirds of non-citizen immigrants who indicated that they were asked to show photoidentification reported that they went on to cast a vote. A potential response to the inefficacy of photo-id at preventing non-citizen voting is found in laws recently passed by Kansas and Arizona that require voter registrants to prove citizenship.