There has been plenty of time to destroy the evidence. Whe the GOP poll watchers were in front of a judge or complaining outside, the votes for Romney were being destroyed and taken out with the trash. There’s no way Romney got less votes thatn mccain.
I'll give you a scenario that may work (not saying it's absolutely certain):
2.5 million people die in the US every year. Most of them are old and white (mostly R voters). They are being replaced with young non-white voters (>70% D voters).