I often get asked how Monarchy would be plausible in a nation the size of America. It wouldn’t be, at least not in the aspect that the inquirer is implying. America is not a nation in the true sense, it is a country, but made up of many nations. Furthermore, America is an Empire, hence the title Untied States. It is a massive conglomerate of many different nations, providences, cultures, and states. For monarchy to be plausible in America, while maintaining a type of unity, two things must happen. It first must be broken up into several respective nations, and secondly, for the whole of America to be united it would have to be under an imperial system. What this means is that the country would be split into multiple nations, based off whatever metric would be best used to define each nation. Each one of these respective nations would have a Sovereign (King/Queen/Principality) and this Sovereign would rule his nation with the customers that best suit his people. To maintain the empire however, there would have to be an Emperor. This would be a Sovereign that transcends all the lesser nobles and kings. Each individual ruler of the multiple nations would swear his loyalty to the Emperor. What this allows is for each nation to be ruled within the context of what is best for those particular people (most likely a dominate ethnic group), while at the same time uniting the different nations under the banner of one Empire. Furthermore, it is advised that there be a specific religion adopted, preferably High Church Christianity (obviously for me it would be Catholicism). This creates a metaphysical glue that unites each nation by a shared faith and helps negate any confrontation based off more petty differences.
Of course there would be much more to it, but in a very brief and simplistic nutshell, that is how Monarchy would be workable within Americas geographical setting if one sought to keep it all together under a single Empire. However, there are several problems that lay in the path, such as Americas liberalistic attitude, history of sedition, contempt for authority, contempt for monarchy, extreme degrees of multiculturalism, and hatred toward Catholicism, Orthodoxy and to some extent High Church Protestantism. Its not something I see happening any time soon.