UPDATE Nov 8: As the dust clears, the race looks like a landslide in the battlegrounds. President Barak Obama has won all nine of the closest toss-up states, bar North Carolina.
His swing state sweep has given the Mr Obama a 303-206 lead in the Electoral College tally that decides the presidency, set to become 332-306 if he is confirmed in Florida (scroll down for state-by-state results)
As West Coast votes were counted, the incumbent also opened a slim lead in the popular ballot with just under 60 million votes (50.3%) to Romney's 57 million (48.1%).
Click for larger, interactive map.
UPDATE 8PM: Watch Obama's thumping victory speech, delivered shortly after midnight in Chicago (above), and Romney's gracious concession speech in Boston (below):
Black magic exponent statistician Nate Silver, who last night said Obama had a 92% chance of victory and correctly called the winner in 49 of 50 states, discusses his craft, and tonight's result.
6.55pm: Mitt Romney is delivering his concession speech.
Click table for larger, interactive version. Source: RealClearPolitics.com
5.19pm: CNN, NBC, ABC, Fox News and the Wall Street Journal have have called the race for President Barack Obama.
Seconds later, Obama tweeted "Four more years" and a photo (above) of him hugging his wife Michelle (according to Forbes, it quickly became the most re-tweeted tweet of all time).
Ohio tipped the incumbent over the top. He may extend his lead if he takes states still open, including Florida where we holds a narrow lead.
5.07pm: According to an ABC News report, former Florida Governor Jeb Bush has told the Romney camp they have lost the swing state (officially still in play with Obama leading 49.8% to 49.3% with 94% of votes counted.
5pm: US pundits scratching their heads over Mitt Romney's poor showing do worse than check out Rod Vaughan's Doubts over Downton Abbey drive American voters to polls.
During the first presidential debate, the Republican challenger threatened to cut funding for the Public Broadcasting Service (PBS), which screens the show in the US, along with favourites like Sesame Street.
Not amused by Romney's proposed Public Broadcasting Service cuts.
4.50pm: With the Democratic West Coast including giant California yet to be tallied, Romney is struggling to keep his head above water. Networks are likely to call Florida and Ohio for Obama in the next hour, sealing the Republican's fate.
4.45pm: A storify feed is aggregating photos from US Embassy parties around the world. Sadly there's still no picture from the US Embassy in NZ's Chicago Bar party, which our man in Wellington describes as a "sensory overload." But here's the US Embassy Bangkok party:
4.30pm: With Romney struggling, the silver lining is that Republicans are set to hold a clear majority in the House of Representatives, as expected, setting up Obama for a deadlocked final term.
But Republican hopes of taking control of the Senate seem a stretch at this point. The party needed to pick up four seats; at this point it has lost two.
4.15pm: The situation continues to deteriorate for Romney. US networks are projecting he will lose another swing state, New Hampshire. Fox News pundits have started to talk about recounts. Time's Swampland blog reports the Romney victory party HQ in Massachusetts has turned off its live feed and switched to campaign videos.
The Republican challengers small consolation is that he continues to lead the popular vote. But with the largest (by population) state of California still to be counted, and other West Coast states yet to come, that could also slip away.
4pm: Romney is on the ropes.
US networks say he has lost the swing state of Wisconsin.
He is also projected to lose the swing state of Pennsylvania, and is behind in close Florida and Ohio races.
SENATE (ONE THIRD OF 100 SEATS UP FOR ELECTION)
Republican: 42 (pre-election day: 47; 37 not contested this cycle)
Democrat: 45 (pre-election day: 53*; 30 not contested this cycle)
* including 2 independents who caucus with the Democrats.
3.45pm: In Massachusetts Democrat candidate Elizabeth Warren, rated an outside chance for the Senate, has defeated Republican incumbent Scott Brown according to an NBC projection.
The Democrats have now made a net gain of two in the Senate. Republicans – currently down two – need a net gain of four to take control.
3.30pm: NBC and Bloomberg have called Pennsylvania for Obama. The Romney campaign had made a last minute push into the swing state.
3.15pm: Republicans, who need a net gain of four to take control of the 100-seat Senate, have lost the race in Maine. Retiring Republican Olympia Snowe will be replaced by Independent candidate Angus King, who is expected to caucus with the Democrats.
3pm: With more than half the votes counted, Romney is a faction behind in the key battleground state of Florida. He also trails in Ohio.
2pm: In a move that may assist Obama in Ohio, bailed-out car company Chrysler has given all its workers the day off to vote. The automaker's executives have criticised Romney for an attack ad that said the company was shifting some assembly to China.
1.45pm: AP has called three states, putting conservative Kentucky and West Virginia in the Romney camp, as expected, and liberal Vermont for Obama.
1.30pm: With cautious, goody-two-shoes networks only releasing issue-based exit poll result questions, all eyes are on the raw vote count.
With just 4% of the vote counted, Florida is achingly close as predicted with Obama on 1,296,710 (50.2%) and Romney 1,270,981 (49.2%).
1pm: Gamblers continue to back the incumbent. Ladbrookes has Obama at 1/5, Romney at 7/2.
11.45am: Polls close over the next two hours in three key battleground states that will likely decide the race. Expect US media to release exit polls as soon as the ballot closes in each state. And the exit polls in these three states may well indicate the outcome of the Presidential election.
Florida polls close between 7pm and 8pm US Eastern Time (1pm - 2pm NZ Time.
Virginia closes at at 7pm ET /1pm NZT.
Ohio closes at 7.30pm ET/1.30pm NZT.
11am: Dateline Auckland: John Dybvig, quietly going about his business, is one of an estimated 11,000 US ex-pats in NZ who are entitled to vote in the presidential election.
10.30am: A Pennsylvania official has confirmed to Computerworld that an electronic voting machine did change a vote for Obama to a vote for Romney (the voter concerned filmed the incident on his cellphone as he grew frustrated with the machine, then later posted the video to YouTube - see below). The machine was taken offline to be re-calibrated.
The Wall Street Journal has reported long lines, and multiple problems with electronic voting including server crashes.
If the result is tight, it seems electronic votiing systems will provide furtile ground for legal challenges.
FINAL BATTLEGROUND POLLS
Click to zoom. Source: Politico.com
The race has been the most expensive in history. For the first time, both candidates refused public funding, and the spending cap that comes with (in 2008, Obama declined public funding; McCain accepted public funding). Obama, Romney and their proxies have spend a record $US2.6 billion on the election.
The Presidency is decided by the Electoral College, which system gives each state a certain number of votes based on population, with 270 needed to win. The most populous state, California, gets 55 votes, while several small states have just 3.
Given Obama's lock on the East and West Coasts, and Romney's lead in most of the South, the race comes down to a handful of battleground states - most of them in the Mid West.
There are paths for Mr Romney to reach the magical 270 mark without winning the largest Mid West battleground, Ohio (see the New York Time's Paths to Victory graphic), but they all require an upset win in several states where Obama is regarded as safely ahead
No Republican has ever won the White House without carrying Ohio, but Politico's latest poll-of-polls has Mr Obama ahead by 50% to 47.1%.
Another factor: although polls show a neck-and-neck nationwide race, they also show that a clear majority of Americans now think Mr Obama will win - and for better-or-worse, last-minute undecided voters often break for he candidate they think will win in their desire not to have backed a loser.
The New York Times' FiveThirtyEight Forecast gives Obama a 92% chance of winning (again assuming swing state polls are accurate). Nate Silver, who writes the blog, says some polls could be rogue, but with 22 out of 23 in close states favouring the incumbent, "it's beyond a coin toss."
Not everyone believes the polls are correct. Ironically (given a Wall Street Journal/NBC/Marist poll puts Mr Obama ahead nationwide, and 51% to 45% in front in the swing state of all swing states, Ohio), one Wall Street Journal board member warns polls are Democrat-biased, and says Mr Romney will win.
In the event of a 269-269 Electoral College tie (which has happened twice before), the House of Representatives will decide the President (presumably picking Mr Romney if it the Republicans maintain control) while the Senate will decide the Vice President (if the Democrats manage to maintain their majority, they will pick Mr Obama's running mate Joe Biden).