He’s allowed to campaign from prison if convicted, too. Eugene Debs did it, after all. Logistically, I’m not sure how it’d work with a former president. If he’s ever locked up, it’s going to have to be in a supermax, for his own safety. The Unabomber’s old cell in ADX Florence will be the perfect fit for the needs of a former president and fellow Ivy League graduate! But at any rate, there’s no law stopping him from campaigning by mail.