It's not just the re-use and sharing of lists, but also the incredible Facebook-style targeting available to anyone for spamming. Anyone can sign up for something like ActionNetwork.org or NationBuilder and send out an email blast to registered voters in a particular zip code. NGP VAN is even more powerful.
The whole industry is mature and super targeted like any other spammer, but mostly immune to spam regulations (because politics are specifically exempt from CAN-SPAM etc., and most voter registration and donation data is public record). The whole pipeline is thoroughly automated and you're marketed and remarketed to just like you are with Google or Amazon, but without any of the already-minimal consumer and privacy protections.
Good.
If the price of living in a democratic society with transparent voter/donor records is a few annoying emails, we should all be paying that price gladly.
Their targetting is shit. The people selling the targetting capability are scamming everybody else. I get countless spam messages from both political parties, both seemingly certain that I support them. I never donated to any of them.