US election swing states explained: where do Hillary Clinton and Donald Trump need to win?

This could be the closest presidential election result since George W Bush vs Al Gore in 2000 - when it came down to one state and a few hundred votes - and certainly the nastiest campaign since John Quincy Adams vs Andrew Jackson in 1828.

Back then Jackson's wife was accused of adultery and died before her husband was inaugerated. Jackson blamed his opponent for her untimely death.

According to latest polls, Hillary Clinton and Donald Trump are neck and neck in the race for the White House - the former's lead slashed in recent days by fresh allegations around keeping emails on a private server by the FBI.

Every election the so-called swing states are seen as the make-or-break of White House aspirations and 2016 will be no different.

Mrs Clinton has been campaigning in the key battleground states of Florida, North Carolina, and Pennsylvania. Mr Trump has been targeting Democrat leaning states like Wisconsin and New Hampshire in an attempt to win over undecided voters.

But what are the battleground - or swing states, and why are they so important?

To understand that, one must first understand how the US voting system works.

Each of the 50 states gets a certain number of Electoral College votes, based on the number of senators and representatives in those states.

There are two senators in each state, while the number of representatives is determined by the state's population.

For example: the highly populous California gets 55 Electoral College votes, whereas Alaska only gets three.

Each of California's 55 electors will pledge their vote to the presidential candidate who gets the most votes by citizens in that state.

The first candidate to reach 270 Electoral College votes is elected president.