If I were tampering with voting, I would do it on the front end, not the back end. That's because if the code is ever discovered on the back end, it is damning. A misaligned or badly proportioned mapping from touch coordinates to ballot options is much easier to explain away as an error or incompetence.

Doing it at the front end won't flip as many votes because many people will see the incorrect registration and stab at the touch screen a few more times until they find the right spot. But some will not notice, and in a close race that could be good enough.