To win this election, Mr Donald Trump needs to hold on to the states which voted Republican in the last election and swing more states over to the red column. Mrs Hillary Clinton has the less daunting task of holding on to the states that voted for President Barack Obama in 2012, while also making a bid for a few traditionally red states. Melissa Sim looks at the state of play.
Acknowledging the importance of keeping it in the blue column, the Democrats held their National Convention here in July. The Democrats have a 92 per cent chance of winning the state, which voted Democrat in the last three presidential elections, according to The New York Times. But Mr Trump is hoping to make inroads here because the state has a comparatively higher proportion of older white voters without college degrees - a description that fits his voter base.
The phrase "As Ohio goes, so goes the nation" refers to the fact that no Republican has ever won the White House without winning this state.
As for Democrats, the last person to win an election without taking Ohio was Mr John F. Kennedy in 1960. The Republican National Convention was held in Cleveland in July, an acknowledgement of the state's key role in the race.
According to The New York Times, Democrats have a 69 per cent chance of winning the state, which voted Democrat in the 2012 and 2008 presidential elections, and Republican in 2004.
It has a high proportion of Hispanic and Latino voters, but they do not vote as a bloc. The state is always a battleground because winning it means sweeping 29 electoral votes - the same as New York - which is the third-highest number after California and Texas. The New York Times predicts that the Democrats have a 71 per cent chance of winning the state, which voted Democrat in the 2012 and 2008 presidential elections, and Republican in 2004.
On paper, it should go to the Democrats because of its racial diversity. The Democrats also have more registered party members than Republicans. But it also has one of the lowest rates of college education, which gives Mr Trump an edge. The New York Times believes the Democrats have a 70 per cent chance of winning. The state voted Democrat in the 2012 and 2008 presidential elections, and Republican in 2004.
It voted Republican in the 2012 and 2004 elections, but swung to the Democrats in 2008. According to The New York Times, this is a swing state, where Democrats have a slight edge of 66 per cent.
Republican candidate Mitt Romney won here in 2012, and Mr Trump must hold on to the state. But the demographics may not be in his favour. The state's urban centres are growing, and many of the inhabitants are educated, young and relatively wealthy - a demographic that favours Mrs Clinton.
It voted Democrat in the 2012 and 2008 presidential elections, and Republican in 2004. The New York Times has categorised it as a toss- up state, with the Republicans having a slight edge - a 56 per cent chance of winning. Mr Trump actually lost the state to Texas Senator Ted Cruz during the caucus, while Mrs Clinton won the state only by a razor-thin margin against Vermont Senator Bernie Sanders.
It voted Republican in the last three elections, and The New York Times predicts that Republicans still have a 64 per cent chance of winning it. However, the paper considers the state a swing state - one that Mr Trump cannot afford to lose. To win it, Mrs Clinton must continue to get out the vote among minorities.
Most forecasters believe Arizona still leans towards the Republicans because of the way it voted in the last three presidential elections, but some are now categorising it as a toss-up state. This is not a traditional battleground state, but Democrats are making a play here and some poll averages are showing a statistical tie between Mr Trump and Mrs Clinton.
There is still a 70 per cent chance the state will vote Republican, according to The New York Times.