ThHey have presidential form of government and the president is directly elected by the government. Once elected, the President appoints his own team to run the government. The Secretary of State is the closest equivalent to the Prime Minister and the present Secretary of State is Mrs. Hillary Clinton.
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America haas no existence as such except in geography where we have North and south america.
What we have as a soveriegn country is a conglomerate of states HAVING GOVERNORS AS THEIR HEAD....unified under a single Presidential form of government and known as United States of America.
As there is no post of PM as such there...so no one.