The outcome of the US election is likely to be decided by a small number of states that are not decisively Democratic or Republican in their support for the presidential candidates.
US presidents are elected under a system known as the Electoral College. The college holds 538 electors, divided among the 50 states and the District of Columbia according to the number of US Senate and House representatives each state has in the national Congress.
Forty-eight states have a winner-takes-all rule; whoever earns the most support at the ballot box takes all the electoral college votes on offer in that state. Nebraska and Maine are the only states where Electoral College votes are allotted as a proportion of the popular vote.
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A presidential candidate must get 270 Electoral College votes to win the election, just over half the total amount. Romney and Obama can each rely on certain states going their way - but in a few other states the outcome is far less certain before polling on November 6.
"We have states that reliably vote Republican, like Texas and South Carolina, and we have states that reliably vote Democrat, take Massachusetts for instance," says Dr Melissa Miller, associate professor of political science at Bowling Green State University.
"Then we have states like Ohio and Florida, who are more balanced in terms of their partisan composition. That means they can be up in the air - if the wind is blowing Democratic, they'll vote Democratic. If it's blowing Republican, they'll vote Republican. They are called swing states because they swing back and forth."
States in play
In 2008, nine states were considered to be swing states as voters decided whether to choose Obama or his Republican rival John McCain. In the event, Obama took all of those states, cruising to 338 Electoral College votes. But with high unemployment and the poor state of the US economy uppermost in people's minds this year, Romney is expected to push Obama much closer.
"He's going to get more of a challenge from Romney for a couple of reasons - number one, the US economy is in a terrible shape," says Dr Jason Johnson, Professor of Political Science at Hiram College.
"People are unhappy so he's not going to win by as much as in 2008. There are some states that he just barely won in 2008. He is not going to win North Carolina this time. In 2008 there were a couple of states that Obama won with just 50 or even just under 50 per cent of the vote, and so Indiana, North Carolina and Missouri, which were called swing states in 2008, these are not swing states now. Obama is not going to win those."
Peter Fenn, a Democratic strategist, says both parties are concerned about a dozen tightly contested states, but that they are paying the most attention to three states which together hold 11 per cent of the 538 national Electoral College votes.
"You've got Ohio - which is always a swing state. Then you've got Virginia, which has recently become a swing state, especially after 2008. And you've got Florida, another big prize," he says.
Republican strategists also consider Ohio, Florida and Virginia the most important swing states to win this year.
"I think it's very hard to see Romney win without Virginia," he says. "It's all guesswork right now, but what I can say with confidence is that Ohio and Virginia are truly critical for Romney," says Matt Mackowiak, a Republican strategist.
Republican campaign managers are looking back to John McCain's failed presidential bid in 2008, re-examining why he lost out to Obama across the swing states and how the Romney campaign can do things differently to wrest back control of them.
"One strategy has been elucidated by [leading Republican strategist] Karl Rove. He says that the most direct, most likely and easiest path for Romney to get his 270 Electoral College votes is to take the McCain map from 2008 and then add on what's called a 3-2-1 strategy," Mackowiak says.
"The 'three' is Indiana, North Carolina and Virginia, the 'two' is Ohio and Florida, and the 'one' is any other state that Obama won last time that the Republicans can flip. If you do that, you can get your 270."
No one is predicting that Obama will win at a canter, as he did four years ago. With the US economy sputtering and unemployment stubbornly high the Romney campaign now sees opportunity in the swing states - and its fundraising effort is accelerating. The Romney campaign brought in $76m throughout May, compared to $60m raised by the Obama campaign.
"In 2008, Obama had such an enormous financial advantage over John McCain that he was able to advertise not just in swing states on television, but he was also able to pay staff in field offices right across the swing states," Miller says.
"John McCain just didn't have the money to put boots on the ground in the swing states. Now the writing is on the wall that Romney is not going to have that kind of problem, and his fundraising is beginning to take off in ways that suggest he won't be behind in that kind of ground game."
The Obama campaign in 2008 set the standard for a new type of political campaigning that proved decisive in the swing states. It poured millions of dollars into contacting campaigners and voters through email, social networks, and online photo and video websites. It left the McCain campaign looking flat-footed, unresponsive and old-fashioned. But with Obama's "Hope" message now ringing hollow for millions of unemployed Americans, his campaign has a harder sell.
"The level of enthusiasm for the Obama campaign in 2008, when he had four million donors, 15 million email addresses and 13 million volunteers - it may be hard to reach that level of activity this time," Fenn says.
Republicans organising Romney's campaign are au fait with direct campaigning and how it can be used to chip away at Obama's vulnerabilities. Republicans organising a recent recall election campaign of Wisconsin's governor Scott Walker assiduously contacted millions of people and helped him defeat Democrats and labour unions ranged against him.
"The real revolution is the online revolution. It's totally changing the game in campaigns; you can now target an individual voter with a message that in the past you would have targeted to a broad audience," Mackowiak says.
"Both campaigns and their allies on the outside are going to have more money that any race in history, but the technology has changed in such a way that you're now able to target voters with so much more granularity and specificity than ever before."
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The campaigns are now spending more than ever before in small- to medium-sized media markets. Places like southern Nevada, Cedar Rapids in Iowa and Norfolk in Virginia are being targeted in a bid to snare votes that could eventually decide who becomes president. Meanwhile, the airwaves are also being flooded by ads from well-funded Super Political Action Committees, ostensibly independent committees that campaign on behalf of the presidential candidates.
"When a campaign doesn't have a lot of money to spend, they'll put what they have into the biggest media markets, where they can get the most bang for the buck. Now the two candidates will go after every possible voter, with the campaigns reaching into areas they would otherwise have not," Miller says.
"Presidential elections are high-information environments. Even if you are someone living in a sparsely populated place, you're still going to be getting a lot of information about the campaigns, through cable and internet, and so forth. Then the ground game becomes important - in terms of who can be out there calling voters."
Romney was buoyed by Scott Walker's gubernatorial win in Wisconsin and his campaign is confident he'll push Obama much harder in the swing states than McCain did. But the injection of funds is not a vaccination against possible complacency.
"Obama won by such a huge amount in 2008. He could spot Romney 30 electoral college votes and still win," Johnson says.
"When you have an incumbent, you have two important questions - number one: 'has the incumbent done a good job, yes or no?' And then question two is 'do you think the challenger can do a better job'? That's what we are looking at in these swing states. You have plenty of people who say they aren't happy with the job Obama has done, but that's not the same thing as saying Romney can do better. Barack Obama's losses do not automatically translate into Mitt Romney gains."
The campaigns now have about a month to refine their plans before Labour Day on September 3, after which point the race for the presidency will begin in earnest. They'll have the attention of millions of Americans who are back from their summer holidays - and face the scrutiny of a huge number of undecided swing voters who have the power to decide who will be the US president.