then a good rationality test is whether you would take that offer.
In which way is it a good rationality test if you have no idea about my utility function?
Unless you have a really weird utility function that values voting in and of itself, what matters is the outcome of your vote. If the predicted outcome of changing your vote is really small, say 0.00000001% chance of being the tie breaking vote, otherwise nothing changes, then the utility of your vote should be near zero.
If there's such a small chance of your vote making a difference in the election, you should be comfortable voting for trump.