I've been reading a couple of biographies of Founding Fathers recently, and I struck by how frequently major leaders of the Revolution voiced support for the King in the years leading up to the revolutionary war. It makes a lot of sense that many of the opponents of unfair taxation backed the King hoping that he would be a check on parliamentary power. What I don't understand is how leaders like Hamilton are praising him in 1774 and condemning parliament, yet by 1776 the Declaration is almost exclusively targeting the King as the reason for all of the issues between the Colonies and England. What causes this major shift in opinion in the span of only a couple of years?
There are a few reasons for this, mostly steaming from England's political turmoil in the 17th and early 18th centuries. During this time, it was difficult to attack the king directly. Monarchs tended to be violently sensitive about attacks on their person, particularly after Charles I lost his head. It was safer in just about every way to attack the monarch's ministers.
Attacking the king's ministers and advisors was actually more appropriate than attacking the king himself. Since Robert Walpole placed himself as an intermediary between the German-speaking George I and the Parliament, more and more of the actual process of governance rested in the hands of Parliamentary leadership, with the king only setting broad policies and approving or disproving of appointments and Parliamentary acts. By the time of the American Revolution the prime minister (to use a term that didn't quite exist yet) wrote the king's annual address to Parliament - including the one where he announced the signing of the peace treaty with the Americans, a measure he vehemently opposed and threatened to abdicate before he allowed it.
If the press painted the king as a well-meaning ruler surrounded by corrupt and self-interested advisors, they did so because this was largely the case. The king may also have appreciated having public fall guys that could be fired at will for mistakes and failed policies.
On the colonial end, keeping attacks focused on Parliament and the ministers rather than the king left open a path towards reconciliation within the British imperial system - the desire of most Americans (almost certainly) before 1776. After the bloodshed around Boston in 1775 and the king's employment of Hessian mercenaries in the massive army he sent to New York in '76, radical Patriots had an opening to attack both George III specifically and the institution of monarchy generally, and to have a large enough percentage of Americans be receptive to that argument to sustain the revolution.
Texts:
Edmund S. Morgan, The Constitutional Origins of the American Revolution
Thomas Fleming, The Perils of Peace