I don't understand this particular use of the gerund yet. To me, such a phrase (the first) sounds more like a person is already voting for John Doe, in the present, not that they will do it in the future. Please explain it to me.
The Purdue Owl 1 identifies such constructions as "the future in relation to the present":
For example, I'm hungry, therefore I'm going to eat.
Consider also these examples of future tense:
I'm going to France in May.
I'm having dinner with Jose on Thursday.
I'm going to college in September.
It depends where the person who is saying this is at.
Before going to the voting booth -
I will vote for John Doe.
In the voting booth speaking to someone else (probably not legal) -
I am voting for John Doe.
However this can also express an intention before voting, meaning -
I am expecting/planning to vote for John Doe.
Later that day -
I voted for John Doe.
Only the last one is past tense.