THE RACE TO 270
The president is not elected by popular vote but by the Electoral College. The number of Electoral College votes per state is roughly proportional to the population of that state. California is the biggest with 55 votes. There are 538 votes in total, so to secure the White House a candidate must win 269 votes plus one for a majority.
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Obama and Romney make last-minute pleas
US President Barack Obama and Republican Mitt Romney made last-minute pleas for votes.
The Democrats began the election with an advantage because many of the most populous states, such as California and New York, are strongly Democrat. According to analysis by the Real Clear Politics website, Democrat President Barack Obama began the campaign with 201 likely votes, Republican challenger Mitt Romney with 191.
By the measure above, this leaves 11 states as contestable; of these, the most valuable are those with the most electoral votes in which the contenders are closest. This year, Ohio, Florida and Virginia are the most crucial, along with North Carolina, Pennsylvania, Wisconsin, Iowa and New Hampshire.
This also means it is possible to lose the popular vote and win the election. With Mr Obama leading in the Electoral College count so far, but tied in the national polls, many suspect this could happen this year.
Americans are not just voting for their president today. About a third of the Senate is up for election, as is all of the House of Representatives. Early predictions suggest Democrats will maintain their control of the Senate, Republicans the House, prompting fears of further gridlock, whoever wins the White House.
TRANSFER OF POWER
If Mr Romney wins the election, he will immediately become the president-elect, but will have no formal executive powers until he is inaugurated. He will be inaugurated and take the oath of office in Washington, DC at noon on January 20, 2013. Between the election of a new president and the inauguration, the president-elect will traditionally name a transition chief to manage the administrative tasks of the transition of power. A White House Chief of Staff will be named and cabinet secretaries will be nominated, though they must be approved by a majority vote in the US Senate, which cannot happen until the new year.
STATE BY STATE
Americans will also be voting on various referendums, as well as for state, county and even school board positions. For example, Arizona's Sheriff Joe Arpio, famous for his enthusiastic targeting of suspected illegal immigrants and fondness for chain gangs, is up for election.
It is estimated it will take some people 45 minutes just to fill in their ballots.
Each state has its own election laws and regulations, some which make it quite difficult to cast a ballot. Lawyers are already circling in Florida and Ohio, where early voters have been waiting in lines for hours only to see doors closed ahead of them.
New voter identification laws that critics claim benefit Republicans (because voters from minority groups, who are more likely not to have photo ID, are also more likely to vote Democratic) have also caused controversy.
Defending the laws became harder after the Pennsylvania state House Republican Leader Mike Turzai listed as one of his achievements: “Voter ID, which is gonna allow Governor Romney to win the state of Pennsylvania, done.”
In Iowa, where this election began with the Republican caucuses in the snows of January, the counting of early votes began in late September, followed days later by other swing states, such as Ohio. Iowa began counting absentee ballots on Monday, the day before the national election. Well over 600,000 Iowans have already voted, a new record for the state, with a plurality of those votes coming from registered Democrats.
POLL OPENING TIMES
Each state sets its own hours for when voting polls are open, but the earliest will begin at 6am (10pm Tuesday AEDT) in Maine, New Jersey, Connecticut, New York and Virginia on the east coast. Polls typically close between 6pm and 8pm, so the eastern states can begin counting after 10am Wednesday AEDT. Judges will order polling places to remain open for longer if there are delays in voting, such as very long lines or malfunctioning equipment.
The media will not announce the winner of a state until the polls in that state close. Exit polls should not be treated as reliable – they have missed badly in the past. Considering how close many states are – and the embarrassment media organisations have suffered from calling a state early and then having to rescind that prediction – expect to not know who has won the election until late into the night.
Most likely, there will be no announcement until after 10pm Eastern US time (2pm AEDT), and it could be much later – perhaps into the next morning.
Gay marriage is a hot issue in the US. Four states – Maryland, Maine, Minnesota and Washington – will have that issue on the ballot as state referendums. Gay marriage is already legal in Maryland, but the referendum could overturn it. Maine is voting on whether to allow gay marriage for the second time in just a few years and Minnesota is voting on whether to adjust the state constitution to make marriage strictly between a man and a woman.
Key Senate races to watch are in Indiana, Connecticut, Maine, Massachusetts and Missouri. In Indiana, right-wing tea party favourite Richard Mourdock ousted moderate conservative Richard Lugar, a Republican icon, in a bitter party primary months ago. Had Lugar won the GOP nomination he would have held this seat for the Republicans easily, but Mourdock is considered too extreme by many, even in this conservative state.
In Connecticut, former World Wrestling Entertainment CEO Linda McMahon is a conservative Republican trying to win the seat of the retiring Joe Lieberman, Al Gore's running mate in 2000. Democrat Chris Murphy is trying to hold the seat for the Democrats and is now favoured slightly.
In Maine, the Republican and the Democrat are both trailing Angus King, an independent candidate and former governor. Although King has no formal ties to either party, he is generally considered more liberal than conservative and the Democratic candidate, Cynthia Dill, running a very distant third place because so many Democrats are voting for King.
In Missouri, Republican Todd Akin, a darling of the tea party and an ardent opponent of abortion, was favoured to take this seat away from the incumbent, moderate Democrat Claire McCaskill. However, Akin turned the race upside down when he defended outlawing abortion even in cases of rape, declaring that the female reproductive system has ways of preventing a pregnancy in case of rape. This is a conservative state that will go for Mitt Romney, but McCaskill is now favoured to keep the seat for the Democrats.
In Massachusetts, moderate Republican Scott Brown won a special election in 2010 to replace the late Senator Ted Kennedy, younger brother of the former president and an icon of American liberals. In this deeply blue state, even a moderate Republican like Brown was always going to have trouble and he's found it in Democrat Elizabeth Warren, a Harvard Law professor and sharp critic of Wall Street shenanigans. Brown led in most polls throughout the summer, but Warren has now pulled ahead in most surveys and is favoured to win a narrow victory, returning this seat to the Democrats.
SITES TO WATCH FOR NEWS
You will be able to read constant updates from this website's live election coverage starting at 6am. Alternative sources in the US for specialist coverage include:
Taxes and budget
Mitt Romney wants to cut taxes 20 per cent across the board and pay for the measure by closing unspecified tax loopholes. Barack Obama wants tax rates for wealthy Americans to rise, returning to the levels they were at during the Clinton administration, but keep the tax rates for everyone else stable. Mr Romney wants to cut the budget deficit through massive cuts in government spending, though he has not said what he will cut. Mr Obama wants "shared sacrifice", which means a mixture of budget cuts and tax increases for the wealthy to reduce the deficit.
Mr Romney wants to increase defence spending – about the only part of government that he thinks is under-funded, while Mr Obama wants a gradual reduction in defence spending as he continues his plan to wind down the war in Afghanistan. Mr Romney opposes Mr Obama's plan to end the war in Afghanistan, just as he opposed Mr Obama ending the war in Iraq.
Mr Romney opposes abortion rights and gay marriage and promises to use the power of the federal government to limit or end them. Mr Obama has come out in support of gay marriage and promises to protect abortion rights and contraception. Since the president appoints Supreme Court justices – and those justices decide on the constitutionality of laws covering abortion, gay marriage and many other issues – a president has the ability to shape national policy in those areas for many years after he leaves office.
Mitt Romney opposes the Affordable Care Act ("Obamacare"), the signature legislative achievement of the Obama presidency and one that will guarantee health care access to almost all Americans. Obamacare is modelled very closely on a similar law in Massachusetts signed into law by Mr Romney when he was governor of that state, but Mr Romney says what was suitable for his state is not suitable for the nation as a whole. He will make it his first priority to undo the law.