And that should tell everyone what a scam the duopoly is. They are both nothing more than criminal gangs, yet few in their ranks dare speak out. They are all co-opted by the party leadership and their wealthy donors.
They may be “co-opted by ... wealthy donors” ... but they also feed into the madness of their voter base when that is the easiest way to win, which it often is. They are professional politicians and if they lose their elected positions their careers end, along with their salaries and perks. It is only those in each party who are well placed who get the cushy jobs in corporations even if they lose or resign from Congress.
“Duopoly” is often a useful term when analyzing how the system works overall to fool the people, and it explains why big corporations often contribute to both sides, but it is wrong to use it when concretely analyzing policies, real differences, or when choosing who to vote for.