"in which two revolutions was the growing power of the middle class a major cause of revolt"

There multiple examples in which a growing power in the lower or middle class was a cause for concern and reform. The two most notable examples where the growing power was a major cause of revolt however, was the American and French Revolution.
The American Revolution has its roots within the French and Indian war (or Seven Year's war), which was a global conflict between France and England. The conflict existed on the frontier of the British colonies in America, and after the British won the war they began to heavily tax the colonists whom they spent resources to protect. By this time, there was already a growing middle class from New England through down to the southern colonies. These new taxation were done without consent nor representation for the colonists which greatly angered them as the taxes were already immensely expensive on daily items such as tea and stamps. The enlightenment thought was also influential, as new schools of thought challenged the position of the individual to the state and monarchy. Eventually, sentiment grew and the colonists fought for their freedom from heavy taxation and monarchy, winning it in 1783.
Similarly, the French revolution saw its beginnings in similar fashion to that of the American revolution. France just fought two major conflicts (French and Indian war, as well as siding with the colonists in the American Revolution). Economic hardships was heavily burdensome to the French lower and middle class who were constantly outvoted by the clergy and nobility in the French court, both who did not face as strong hardships as the middle and lower class. The french nobility and royalty grew strongly out of the touch with the middle class majority. Enlightened thought eventually lead the French middle class to revolt and ignite the French Revolution in challenge to the position of 

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