Polling in the key US states will start closing from midnight tonight - Channel 4 News guides you through the crunch times of the presidential battle.

Voting will be taking place today across 51 states amongst around 220 million eligible voters. Turnout for US presidential elections is usually at between 50 and 60 per cent, meaning between 110 million and 132 million American voters are expected to head to the polls (including those who have voted from overseas or have voted early).

The winner will be the first man to reach 270 electoral votes under the US electoral college system. The system, designed to ensure the winning candidate has a not only the popular vote but also a geograhpically diverse one, allocates a number of votes to each state based on its size.

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The states with the most significance under this system are California, a safe seat for Mr Obama with 55 electoral votes, and Texas, a safe seat for Mr Romney with 38 electoral votes.

However, with Mitt Romney and Barack Obama neck-and-neck in most polls going into the election, key to success will be the 110 electoral votes from nine "swing states": Virginia, Ohio, North Carolina, Florida, New Hampshire, Colorado, Wisconsin, Iowa and Nevada (see graphic).

Polling closes first in Virginia at around midnight (GMT) but the first chance of a significant result will come after the closing of polling in Ohio and North Carolina at around 12:30am.

If Mr Obama wins in Florida, as this graphic from the New York Times shows, then Mr Romney would need to win in every other swing state to become president (as long as the other states fall as expected). However, Mr Romney has been leading the polls in Florida thus far.