votar por

Hola Elizabeth

Votar por alguien (to vote for someone) in an election is always por - you can also use "a" to express the same if you are voting for a party, e.g. Voy a votar a Izquierda Unida.

There is no obvious explanation to justify por instead of para with this verb I'm afraid, even though, as you say, one of the uses of the prepostion para is to indicate someone benefiting from an action, which would click here nicely with this verb. 

It is a set expression: votar por alguien/por un partido

Saludos

Inma