Imagine politicians are all grifters, who want to get power and/or parlay revolving door through some deal making inside congress (imagine). If one party ideologically closest to you forces you to eat shit via spoiling votes, you either have to make policy deals or spend your days fighting battles with them (and not making deals to get board membership at some ngo).
The theory of voting third party is very simple:
Imagine politicians are all grifters, who want to get power and/or parlay revolving door through some deal making inside congress (imagine). If one party ideologically closest to you forces you to eat shit via spoiling votes, you either have to make policy deals or spend your days fighting battles with them (and not making deals to get board membership at some ngo).