If they think an election is worth stealing, it's worth voting for the guy they're trying to steal it from. Besides, that forces them to step up their stealing and make it more blatant - which undermine their legitimacy. Rather than giving up, folks need to use the tools they have in order to always make the (legal) moves that the regime doesn't want. That is the way autocratic regimes get undermined.

Here in Europe, they don't even bother to steal elections, because they know they don't matter.