Five days before Americans go to vote, the latest Washington Post-ABC News poll showed the two candidates within decimal points of each other nationally and across the battleground states (49% for Obama, 48% Romney).
And the likelihood of a recount can now no longer be excluded, experts warn, saying uncertainty in the US could have consequences for the wider world economy.
In Europe, the Troika representing the European Commission, the European Central Bank and the International Monetary Fund delayed a report on Greece until after the US vote.
EU finance ministers are meeting on 12 November to decide whether to grant Greece an extra two years to meet its budget deficit targets.
But this schedule could now be called into question if a recount takes place in the US.
Political experts in Washington have pointed to flaws in the American electoral system that could lead to a recount, claiming such loopholes would be unacceptable in Europe.
“For some widely used voting methods, the fail rate, in which a voter’s intent is unable to be ascertained, is well above 2% of ballots cast, which would be an outrage in Europe,” Professor Jeremy Mayer of George Mason University told a journalists.
He added that the fail rate is even higher for absentee ballots, as is the risk of fraud. “Given the huge surge in early voting recently, we don’t even know the fraud rate for that,” he said.
About 22 million people have already voted either by mail – including Americans living abroad. In Brussels and elsewhere across Europe, US expats feel increasingly anxious about the uncertain outcome.
With some 6.3 million Americans living abroad – 1.6 million in Europe – the overseas vote could have an impact in key swing states, such as Ohio, Florida and Virginia. Overall this year, 207.6 million of the 303.8 million Americans are registered to vote, government figures show.
And with the outcome uncertain, the expat vote could prove more important than ever.
Mayer said a number of novelties in the 2012 election are likely to make the outcome even more unpredictable.
Since the 2010 mid-term elections, when Republicans gained control of 20 state legislatures, 11 states have passed new election laws dealing with voter identification, early voting or absentee ballots. Other states are altering their election laws that affect administration.
“Each of these changes increases the chances that there will be unintentional errors, confusion or systemic processing failures,” said Mayer, adding that these elections could result into a no-winner scenario and a possible vote recount.
“The odds of something like that happening have never been greater. And the likelihood that we could resolve it even as well as we did in 2000 is lower than ever,” he said.
In the 2000 presidential race, it took a US Supreme Court ruling to validate Republican George W. Bush's victory over then-Vice President Al Gore.
Some of state laws require voters to produce photo identification. Others curtail early-voting periods that are designed to help working-class people cast ballots if they can't make it to the polls on election day. Others have imposed further requirements on groups that conduct voter-registration drives.
Legal battles over new election laws
Democrats believe that millions of voters could be disenfranchised by these laws and suppress voting among minorities and other groups that tend to vote for democrats. Republicans have said the laws are aimed largely at preventing voter fraud, from illegal immigrants or from multiple voting by the same people.
"We've seen a great deal of litigation in the last two election cycles," said Dan Tokaji, an Ohio State University law professor. "This is shaping up to be an extremely close presidential election in which a lot of these seemingly little things could add up and make a difference in these swing states in Florida or Ohio or Pennsylvania."
Most American states do not have non-partisan electoral administrations. Instead, elected or appointed partisans are typically in charge of elections and that creates additional tension, added Mayer.
Dozens of legal battles are under way in courts across the nation, and some judges have tossed out a few of the laws in politically divided states that could be crucial in deciding the election.
Among those states are Florida and Ohio - two traditional swing states - along with New Hampshire, Virginia, Wisconsin, Pennsylvania and Virginia. They account for 218 electoral votes - nearly 80 percent of the total needed to win the presidency. (See background)
In Florida, new requirements for voter registration drives have led to a dramatic decline in people registering to vote. And in Ohio, a judge rejected a law aimed at cutting back the state's early-voting period.
In Florida, only 11,365 people have registered as Democratic voters during the past 13 months, compared with an average of 209,425 for the same periods before the 2004 and 2008 presidential elections. Meanwhile, the numbers for Republicans show 128,039 registrations during the past 13 months, up from an average of 103,555 during the same period in 2004 and 2008, local media reported.
Another voter identification law in Wisconsin was blocked by two judges. The attorney general has asked the state's Supreme Court to reinstate the law before November, when Wisconsin - home to Romney's vice-presidential running mate Paul Ryan - could be a crucial state in the election.
“Remember the 'butterfly ballot' in Palm Beach? If that one innocent, if catastrophic, snafu in the shape of a ballot had not been made by a local Democratic election official, Al Gore probably would have been elected president,” said Mayer, noting that the electoral laws changes has ignited the fears that the ballots will be tainted.