Surely rationality and morality are completely different animals.
Morality is at once personal, in that I'm sure you could find differences between two monks of the same order if you asked enough questions, but also subjective in that what is considered right and wrong is specific to both time and culture.
But rationality is the degree to which our expectations are logical and based on unbiased experience. So to answer whether an action is rational one must first know what is trying to be achieved.
So it is rational to vote for Hilary if you don't want change, and rational to vote for Bernie if you think that the whole system needs a shake up but neither of those opinions are rational in themselves, they depend in the individual's world view. They can be explained rationally, in terms of why a person has come to hold them, but only retrospectively. There can be rational predictions to expect people to behave according to thier circumstances and to behavioural patterns observed in others but the values themselves are subjective. A rational expectation that someone will hold a certain opinion or value doesn't necessarily mean that they will.
It is rational to expect people to behave in their own self interest because most of us do but that tells us nothing about what self interest is. It doesn't have to be selfish.
Most people on the Left see the common good to be to their personal advantage; if the poorest in the country are educated and looked after when they're sick then everyone benefits from a stronger and more cohesive society. Even a lot of conservatives acknowledge that they don't stand alone, they just differ in their basic understanding of how the world works and what can be achieved.
I think that to claim that there is a rational course of action in the way that you have is to ignore the fact that people have different values and subjective world views.
Voting rationally will enhance the chances of getting the outcome that you desire but that desire is not the result of logic alone.
If it were that simple it wouldn't be any fun.