Need proof? It's all right here, in this Markup analysis of how Gmail is handling email messages from various 2020 US presidential candidates.
The authors seem surprised, though, and I'm a little bit disappointed by that. They really missed the boat by not talking about deliverability and sending reputation. They express surprise that mail from different candidates is treated differently. Those candidates have IP addresses and domains that likely have different sending reputations. There isn't really an email header you add to emails and say "this is political, don't filter it." And I don't think ISPs want one, because they're smart enough to know that if there was such a flag, and if it somehow guaranteed inbox delivery, that it could and probably would be exploited to send mail that many people aren't interested in reading. Recipients would be unhappy, and they're going to get mad at the ISP for not blocking it.
Sixteen candidates. Sixteen different email streams. More than sixteen different campaign managers. Different styles, different methods of attempts at engagement. Some good, some maybe not so good. Different cadence. Different messaging.
Why would you expect an ISP to treat all those mail streams exactly the same? They're not the same. They just happen to be categorized in a group by the authors -- artificially categorized, in that "political sender" is not a category that an ISP applies to certain types of mail.
TL;dr? ISPs don't care who you are or how important your message is. You've still got to play by the rules. And it's a bit of folly to compare two senders side by side and assume they should be treated the same by an ISP. You don't have access to the same reputation metrics that the ISP does, and it's those metrics that govern where the ISP places that mail.
John Levine may agree with me, but 2020 presidential hopeful Tulsi Gabbard thinks it's unfair. Her campaign even alleged that "[Google was going] out of their way to silence messages from [her] Campaign." Her case was dismissed, and while the court never touched on that specific question of spam filtering, they made it clear that Google isn't the government, so you can't bring a first amendment free speech claim against them here.