I can speak from experience that you’ll most likely be asked about your elected representatives so, yes, you should know everyone from your mayor to the President.
Also, you should be ready to give a short 1-3 minute speech about a topic of their choosing, know some current events happening in the country, and answer some general questions like “what do you think is the greatest threat facing America?”
I assume that not every Legion post asks the same questions but those are some of the questions that stood out when I interviewed last year. I came into the interview not knowing anything about the type of questions that would be asked, but already knew what to say based of my interest in politics. If you go into the interview prepared you should be set, but the chances of you being selected to attend are all based off how many other people applied. For example, My legion post was only able to select 1 person/ high school.