THE ELECTORAL COLLEGE
Each state is granted one presidential elector for every member of congress: one for each member of the House of Representatives, where seats are allocated according to population, and two more because each state, regardless of population, has two senators.
That guarantees that each state will have at least three electoral votes. The District of Columbia, home to capital city Washington, gets three electors even though it doesn't have full congressional representation.
In most states, the popular vote winner captures all of that state's electors. The only exceptions are Nebraska and Maine, which award electors proportionally.
The system was written into the US constitution to overcome concerns by small states that they would be left voiceless among their larger-population brethren.
The solution was a middle ground between those who wanted the president to be chosen by congress and others who pushed for a nationwide popular vote.
In what is largely a formality long after the winner has been determined, each state's electors will meet on December 17 in their home states and cast their votes for president and vice president.
Congress will meet on January 6 to conduct an official tally.
The Electoral College system can produce two anomalies. As happened to then-Vice President Al Gore, who lost to George W Bush in 2000, a candidate can win the most popular votes but lose the presidency.
Or candidates can be tied for electoral votes, 269 each, which throws the tie-breaking decision into the House of Representatives.
The choice of vice president falls to the Senate.
With the House expected to remain in Republican hands, a tie would mean a Romney presidency.
Democrats are likely to retain control of the Senate, meaning Vice President Joe Biden would have a second term.
THE STATE OF THE RACE
In 41 states plus the District of Columbia, the results are fairly predictable. States like Texas and Alabama are almost certain to support Romney; California and New York are among those favouring Obama.
An Associated Press tally shows Obama ahead in states with 237 electoral votes; Romney leads in states with 191.
If that holds, Obama must win in enough swing states to accumulate 33 more electoral votes. Romney needs 79 more.
The two biggest swing-state prizes are Ohio, with 18 electoral votes, and Florida, with 29.
Polls show the candidates running even in Florida, but Obama has a slight lead in Ohio. Both states are crucial for Romney.
If Obama wins in Florida, Romney would need to sweep all the other swing states, including Ohio.
If Romney wins in Florida, but loses Ohio, Obama would still be just 15 electoral votes shy of victory. That would leave him with several paths to victory.
For example, he could win by taking Nevada (six electors) and Wisconsin (10), two states where polls show him leading. Because of those stakes, many analysts see Ohio as the pivotal state on election night.
No Republican presidential candidate has ever won the White House without also carrying the Midwestern state.
The other six swing states are Pennsylvania (20), North Carolina (15), Virginia (13), Iowa (6), Colorado (9), Michigan (16) and New Hampshire (4).
THE ISSUES
The economy is by far the biggest issue in the race.
The United States has struggled to return to full strength after the late 2008 financial meltdown and the Great Recession, which began during Bush's presidency.
Obama takes credit for preventing deeper problems and says the economy, while not ideal, is on the road to recovery and that Romney would reinstate the policies that led to the collapse.
Romney says the continued economic weakness demonstrates the failure of Obama's policies and, noting his own record as a successful businessman, says he knows how to stimulate the private sector and create jobs.
Obama this week received high marks, including from Republican critics, for his handling of a deadly storm that devastated the New Jersey shore and parts of New York. But whether this will have any effect on the race remains to be seen.
Polls show that nationwide, Romney holds a slight lead as the candidate best qualified to handle the economy. Nevertheless, Obama maintains his edge in Ohio, where his decision to loan federal money to the failing car industry saved thousands of jobs.
Other major issues include the huge federal deficit, health care and immigration.
Those may be especially important in Florida. The state has a large Hispanic population, and Obama is far more popular with that voting bloc - leaving aside the traditionally Republican voting Cuban immigrant community in the Miami region.
Romney's tough stance on illegal immigrants could hurt him with Hispanics.
Florida is also a haven for retired Americans who rely heavily on Social Security, the federal government pension system and Medicare, the government-run health system for people age 65 and over.
Some older voters might fear that the deficit-cutting proposals of Romney and his vice presidential running mate, Paul Ryan, could lead to reduced benefits.
Florida was also ravaged by the recession, with high unemployment and a collapse of the real estate market.
That economic suffering would seem to argue in Romney's favour. International issues are unlikely to sway many voters.
Obama has pointed to his efforts against terrorism, including authorising the military operation in Pakistan that killed terrorist leader Osama bin Laden, and his withdrawal of US forces from Iraq.
Romney says Obama has been weak with US adversaries like Iran and unsupportive of allies like Israel.
Romney has also promised to crack down on what he sees as unfair trade practices by China.
Obama is America's first black president and Romney would be its first Mormon president, but neither race nor religion has been widely discussed in the campaign.
SPENDING
Money raised in the 2012 presidential campaign has reached more than $US2 billion ($A1.93 billion), a record.
A big reason for the increase this year was the 2010 Supreme Court ruling in the Citizens United case that allowed corporations and unions to spend unlimited amounts in political campaigns.
The ruling reversed a century of US precedent which limited financial involvement by those organisations.
Figures compiled so far show Obama had easily outraised Romney personally, but the president was being vastly outspent by outside sources backing the Republican challenger.
ELECTION DAY
The presidential race is so close that calling a winner could be much delayed.
Polls are not closed in all 50 states until voting ends in Alaska at 10pm Tuesday (1700 AEDT Wednesday).
But in the three states likely to be most pivotal, all votes will have been cast by 8pm EDT (1200 AEDT) Virginia polls close at 7pm EDT (1100 AEDT); Ohio at 6.30pm CDT (1130 AEDT); and Florida at 8pm EDT.
Exit polls will show trends soon after polls close, but, barring unexpectedly strong showings by one candidate, the vote counts in swing states could take hours - or longer.
The 2000 election wasn't decided for weeks because of a dispute over the vote count in Florida.
Bush's victory was ultimately determined by a Supreme Court ruling.
