Since the 2016 Presidential election the 12th Amendment has become a controversial one. This amendment, ratified in 1804, established that a President and a Vice president are elected by the electoral College.
In the 2016 Presidential election Trump was chosen over Clinton by the Electoral College despite the fact that the latter got more votes by the American people. The Electoral college is a 19th century system that is no longer used in current democracies, where the population chooses directly who they want for President, Vice president, etc. It establish an intermediary body of electors that may not represent the people's wish.