Voter fraud has been around as long as voting has. There's a reason the Iraqi's had to dip their thumbs in purple ink when voting after the butcher of Baghdad was deposed.i'm not saying it would be inefficent for a candidate. i'm saying it would be inefficent for an operative. the logistics of impersonating enough voters to matter is difficult.
the video posted above shows someone who got caught for voting 8 times, she got a 5 year prison sentence. that is my biggest point, it is a crime that is only worth it if you are able to do it several thousand times, and the logisitics of doing so are very difficult, and any operation that was sophisticated enough to pull it off would leave hundreds of witnesses and a trail right back to you.
Do a tiny bit of research about the election of Nixon vs Kennedy. The whole thing boiled down to Illinois and ultimately to one county there. A little bit of voter fraud can go a long way. Look at Bush/Gore in Dade county FL if you want a more recent example.