“Will my vote help or hurt my reelection prospects?”
Self interest is the strongest and most basic of human traits.
We are all driven by it, including (or especially) politicians. To expect politicians to somehow be holier than thou is to delude yourself.
What our system lacks is a much more responsive mechanism that would hold the politicians feet to the fire so that they would never stray from seeing their self interest as that of the people that elected them.
I partially agree with you, especially regarding mechanisms to hold politicians’ feet to the fire.
There is altruism in certain persons, including a few politicians who will go against self interest to do the right thing for their constituents and the nation. These typically are the ones who enter government for only a defined period of time to accomplish a specific goal, are not too concerned about reelection, and return home afterward.
We also see people acting in others’ interest in various areas of society, particularly in the armed forces and among spouses & parents — people who would do anything for their battle buddies or family members, including sacrificing their lives.
So yes, self interest is a strong and basic human trait, but is it not the strongest. That is reserved for love, courage, and honor, also intrinsic human characteristics.