The exact number of noncitizens who are voting in our elections is difficult to quantify because of the bureaucratic quagmire perpetuated by federal agencies against the (very few) states that have the resolve to attempt to verify citizenship.
Federal agencies responsible for immigration and naturalization routinely fight efforts to compare voter rolls with lists of known noncitizens.
Yet evidence of noncitizen voting mounts. The American Civil Rights Union just filed an amicus brief with the Supreme Court documenting instance after instance of noncitizens registering and voting. It urges the Court to take up a petition for certiorari filed by Kansas and Arizona seeking to overturn a bad decision on this issue by the 10th Circuit Court of Appeals.