you are correct P-marlowe, it is a serious offense.
And there will probably be federal and state charges against him. And a voter registration is admissible as evidence, however only to prove citizenship in this case (per MrR).
But in the court of public opinion, there are going to be a lot of sheep who believe this.
In this climate, it would not surprise me if Eric Holder or someone on his staff was the guy who forged the document. With Holder as the AG, I don't think any charges will be filed.
This makes Republicans look bad. Therefore it is not illegal.