If you wanted to sway an election with electronic voting you would just have to hit a few points versus going through the hassle of voting in person... Or even voting by mail.
Plus, the problem is lazy, stupid humans. You could have the most secure system on the planet, but if the password is password then it's worthless. Social engineering is a HUGE thing. Look at defcon videos where they "hack" via pretending to be someone in the company and make a phone call. It happens so fast(minutes). Also, there's a video on electronic voting , it might be a vice video or jon Oliver... districts left voting machines easily accessible, not under lock and key, no cameras, nothing...
America should do what Germany does with voting make the ballots easy to read/use, none of this stupid chad shit. There are big circles on our ballots to mark the vote, it's simple/clear.
The xkcd comic sums it up perfectly too.