If I was a member of a political party and wanted to leave, I would probably do so very quietly by cancelling my direct debit and unsubscribing from their emails.
If the situation was heinous, I would probably write a very short-and-to-the-point email giving brief reasons for resigning and send it to them.
What I WOULDN'T do is write a three page letter to the party's HQ and then share it on Twitter instead of emailing them. Especially if I'd done exactly the same thing with another party.
That's mental. But then again, we live among mental people.