It's a bit tricky, maybe, to fire someone in Ontario, Canada because of their politics. With that as the reason. The courts take a rather commonsense approach

It's a bit tricky, maybe, to fire someone in Ontario, Canada because of their politics. With that as the reason. The courts take a rather commonsense approach to wrongful termination. But I do believe it is legal to make hiring conditional on basically anything not prohibited by the labour or discrimination laws. Political affiliation or political belief are not protected categories. Though I can imagine some possibly over-stretched arguments if the political requirement were constructively discriminatory against a protected category. E.g. it's probably not legal to require a devout Catholic to hold a pro-choice view on abortion. Such a rule is nearly the same in effect as a ban on hiring adherents. Other than that, you can be as nepotistic, partisan, or arbitrary as you want. Not American or a lawyer, but as I understand the broad situation is the same in most US states.