What happens is that the campaign you donate to to puts you on their list (allowed) and then shares that list with others in the party (also allowed). They share back and forth so fast you can't get out of it.
This is why it's the email that's shared not the name. FEC records don't have your email attached to to them, but the spam still follows unique emails like "candidate@customdomain.com".
I have a politics label in gmail that is blue/red from 2012 onwards. All the GOP emails are from poking around Romney 2012, and nothing else.
I'd say you'd be surprised on the reuse, but you shouldn't be.
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.
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.