Only US citizens have the right to vote. USCIS is alleging you illegally registered to vote by falsely pretending to be US citizen - a lifetime inadmissibility ground for which there is no waiver (at least one that will be approved.) You'll need to appear at USCIS as requested. Better "lawyer up"at once, otherwise can kiss the USA "bye bye" for the rest of your life.
Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
Part of the process to become a lawful permanent resident and get your green card is establishing that you are not "inadmissible." Grounds of inadmissibility include falsely representing yourself as a U.S. citizen in order to receive any benefit under federal or state law, including the right to vote. There is no waiver for this and only a very narrow exception, so such a misrepresentation leads to a lifetime bar from immigrating to the U.S.
That being said, in Texas at least it was not unheard of for people to be mistakenly registered to vote when they applied for a driver's license without realizing that they had even registered. I have had clients in this situation who have had success in applying for different immigration benefits. Basically, it comes down to whether you intended to falsely claim U.S. citizenship when you registered to vote (or when you were mistakenly registered to vote by someone at DPS, for example, while applying for a driver's license), and whether you can essentially prove that intent to USCIS.
I highly recommend you contact a lawyer experienced in these matters as soon as possible. I am located in Austin and would be happy to consult with you about your situation. Best of luck.