Just looked up what a middle power is: these are the regions that are currently defined as middle powers: Brazil, India, Indonesia, Saudi Arabia, South Africa, and Turkey.It means not tying ourself to the US by purchasing nuclear subs and not being a major player in world affairs in Asia.
Not fighting above our weight division, being a middleweight instead of a heavyweight.
They all flitter between democracy and authoritarianism (many now much closer to authoritarinism). In a world that is dividing along democratic and autocratic lines you clearly want to be in camp democracy and not be a middle power. If there are two many middle powers then democracy loses and this would be catastrophic.