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Yes, that’s normally a good rule to follow, since “whom” is for a sentence’s object, but this is a special case. The clause in question is either a salutation that has no subject or object, and so either “whoever” or “whomever” is correct, or it’s a subject clause (a noun phrase, really) with an unnecessary, stylized “to” for the sake of comedic impact, in which case “whoever” would be correct.
There’s a distressing trend of people (self-identifying “progressives”, for the most part) abandoning family because of ideological differences, so most here will probably tell you to cut contact.
I don’t know you or your father, but as a dad myself, I can tell you that if he has the slightest decency as a parent, he’ll be willing to listen if you come from a place of love and respect. Tell him how you’re personally affected by his actions. Appeal to his paternal instinct. At best, you could convince him to rethink his ideology. If that fails, you can hopefully get him to agree to avoid discussing politics with you.
It’s especially important now, with a nationwide financial collapse looming, for family to stick together.