Well like in the Late 1700's and Early 1800'sSo it isn't already one?
Oh thanks!Take a look at ://en.wikipedia.org/wiki/Emperor_Norton and this version of him from Lucky Luke: https://en.wikipedia.org/wiki/L'Empereur_Smith
Let's say there's some earlier unrest, and the Constitutional Convention chooses the Hamilton Plan, which proposed an elective presidency-for-life, which could easily become hereditary and eventually abolished into a monarchy.
Although they'd probably keep calling themselves President, a la Augustus, or indeed most dictators nowadays.
Just a thought tank for a fic I'm writing!