If I were to wager a guess (and IANL, etc.), he'd need to've committed a crime since he has a permanent resident green card? Not that I wouldn't put it past them to try it, anyway. Though it probably makes for a much more effective propaganda message of, "See; these people are illegally voting and that's why we have to remove them," (just don't look to closely at who he's been voting for).

Ultimately – though –, I dunno; I just know that's what DHS said, according to the article.