The United States is a constitutional republic with all of the executive power vested in the executive branch, which is headed by the President. The President and Vice President are chosen by the citizen voters—although technically electors cast the actual votes.
In a parliamentary system, the Prime Minister is the leader of the party that wins the most seats in the House of Commons and is appointed by the monarch. So the voters in the UK only vote for the Members of Parliament (MP), not the Prime Minister. Therefore, the voters in the US have more say-so over their head of government. This underscores the fact that in the UK the monarch is the sovereign while in the US the People are the sovereign.
While the election and duties of the President are spelled out in the Constitution, the duties of the Prime Minister are conventions that have evolved over time and are not spelled out in black and white. Presumably those duties could be altered, absent the will of the electorate. The US...
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