Doesn't your chance of swaying an election depend on how close it is? If your favored candidate is way ahead or way behind, then changing a few thousand votes doesn't matter. Whereas charity always has some marginal effect.

Also, influencing an election depends on the difference between the candidates. Which not only may be small, but may be difficult to predict, both due to reneging on campaign promises and to specialization - one candidate may do better in a recession, the other in a war. If you pick the wrong guy, your money has negative effect. All of these reduce the effect of spending for votes.

So I think there are a lot of reasons why the effects of spending on elections are diminished compared to spending on charity. But I have a lot of reasons to want to think that, so my opinion should be taken as a summary of one side rather than a balanced evaluation :).