All of these organizations shared (definitely) or sold (assumption) the customized email addresses far and wide. The only other possible explanations for what happened with these email addresses might be that they were stolen during a breach or that a bad actor with internal access shared or sold them. OK, I can buy either of these for one organization here or there, sure. When it happens across all organizations? No, the sharing or selling had to be intentional.
There was no effective way to stop the stream of emails that resulted from a handful of registrations. From my perspective, clicking on "unsubscribe" only served to confirm the email address was good. In other words, perhaps it did unsubscribe you from that specific organization, yet, what you actually accomplished was to provide them with a validated "live target" email they could sell or provide to another organization.
The result was an almost exponential growth of emails. Registering to, if I remember correctly, approximately six organizations led to a constant daily stream of emails form hundreds (did not count them) of groups. Crazy.
I never set out to quantify any of this or run study, so I don't have hard numbers to offer. All I can say is that the explosion of emails was universal across political alignment. Private media organizations also seemed to share email addresses with political party groups and candidates. In other words, I got emails from candidates and campaigns using email addresses I only provided to news organizations (identifiable through plus-addressing).
It didn't take long for me to kill the account and conclude that the iconic "The only winning move is not to play" [1] insight applies.
[0] https://gmail.googleblog.com/2008/03/2-hidden-ways-to-get-mo...