Ah yes i forgot. Everyone who disagrees with you is a leftist.
Yes there is a written rule. It's called the emoluments clause in the US Constitution. So not only is it unethical, it's illegal.
I repeatedly asked you to provide a written rule that says an ex-president cannot support a candidate in a foreign election and you changed the subject every time instead of providing such proof.
So either provide the dawn written rule and i'll admit he did something unethical or if you can't, stop trying to make me say he broke a rule that you just made up.
And for clarification, if Bush Sr, Jr or the ghost of Ronald Reagan tweeted to say they support Andrew Scheer, i wouldn't care either. They're all private citizens with the right to free speech.
I think you don't understand the principle of ethics. ethics and law are 2 different things. in some cases they can overlap, i.e. killing someone is unethical and illegal at the same time, but sometimes they can differ. you can flip off your neighbor for no reason and it's legal, but not ethical. euthanasia is mostly considered ethical, but generally illegal.
doing something illegal means that the police can come after you for it. doing something unethical means you're a POS for having done it, though nobody can call the cops on you. ethics is a code of conduct, unwritten rules if you will.
I never said that Obama's endorsement is illegal. I said it's unethical. I also said that Trump's choice of location for the G7 is unethical. He has backed out of it since by the way, which I find surprising, he's not the type to admit he made a mistake.
I already explained to you how Obama is a public figure representative for the USA. he also has access to white house briefings, which are top secret. so does Bush for instance, who chose to keep quiet, same as Clinton. there are other duties, responsibilities and restrictions due to the fact that he was president. you choose to ignore this and you say I change the subject. this is the subject. just ask yourself for a second why did no other president do so? you think none of them had a preference? I'm sure they did. so why didn't they speak up?
as for the emoluments clause which you brought up, it's a thing. did you know that in Aspen, Colorado you cannot own a catapult? it's just as relevant.
here's a clause in the canadian law which Obama is flirting with again, not illegal. barely legal.
https://www.parl.ca/DocumentViewer/en/42-1/bill/C-76/royal-assent section 282.4