Kamala Harris and Donald Trump.Voting has already begun in the US presidential election - but it could potentially be weeks before we know who has won.The polling stations will open on Tuesday, but millions have already sent in postal ballots in one of the tightest races in living memory.Republican candidate Donald Trump is seeking to return to the White House four years after losing to Joe Biden.For the Democrats, Kamala Harris is looking to create history by becoming the first ever female president.Here is what you need to know.How does the American electoral system work?Each of the country’s 50 states is allocated electoral college votes depending on their size.The winner of a state has its electoral college votes added to their overall tally.With a total of 538 electoral college votes up for grabs, the target for Trump and Harris is to reach 270, giving them an unassailable lead over their rival.In 2016, Trump beat Hillary Clinton by 306 to 232, while in 2020 Biden beat Trump by 306 to 232.What are the key states to watch?The result will ultimately be decided by the results in seven key swing states - Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.Between them, they have a total of 93 electoral college votes.&