WASHINGTON, USA (AP) — The outcome of the hard-fought but still deadlocked presidential race between President Barack Obama and Republican challenger Mitt Romney will be decided tonight by a small percentage of voters in just nine of the 50 U.S. states. That’s because presidents are elected not by popular vote, but under a state-by-state voting system, known as the Electoral College.

The system was born from an 18th century political compromise. States are allocated a fixed number of electoral votes based on population. In almost all cases, whoever wins a state wins all of its electoral votes. And the candidate who captures a majority of the 538 electors becomes president.

Most states are reliably Democratic or Republican, but neither Obama nor Romney has a lock on enough states to win a majority of electors. That means the real battle is for the nine “swing states” — those where the outcome is uncertain. Whoever can carry enough of those states to bring his overall electoral vote total to at least 270 will win.

Nationwide polls, which show the race a virtual tie, do not necessarily predict the outcome. A candidate can win the most votes nationally and still lose by failing to accumulate a majority of electors. What’s important is what happens in the swing states. And, for the moment, Obama appears to have an edge.

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 (6 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 five swing states are North Carolina (15), Virginia (13), Iowa (6), Colorado (9), and New Hampshire (4).

Congress

While attention has focused on the White House race, control of Congress is also at stake. Republicans are expected to retain their House majority. Democrats had been considered in danger of losing their Senate majority, but analysts now predict they will hold it by the narrowest of margins.

A third of the 100-seat Senate will be at stake. To win a majority, Republicans need a net pickup of four seats if Obama is re-elected, or three if Romney prevails (the vice president casts a tie-breaking vote). The retirement of some Democratic senators in conservative-leaning states seemed to open the door for Republican gains. But in several states, Republicans who emerged from the nominating primaries were candidates who, while appealing to the party base, are seen as potentially too conservative or otherwise undesirable for mainstream voters.

Republicans, for example, were counting on wins in Missouri and Indiana. But they ended up all but abandoning their Missouri candidate, Todd Akin, after he remarked in August that women’s bodies have ways of avoiding pregnancy in cases of what he called “legitimate rape.” In Indiana, Richard Mourdock defeated the more moderate veteran senator Dick Lugar, who would have likely cruised to re-election. Mourdock has come under criticism after saying in a debate that when pregnancy results from rape, it is “something God intended.”

Other closely watched races are in Massachusetts, Connecticut, Virginia, Wisconsin, North Dakota, Montana, Nevada and Arizona.

All 435 seats in the House are at stake, but incumbents tend to get re-elected. Democrats seem unlikely to pick up the 25 seats they need to gain control.

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 1:00 am. But in the three states likely to be most pivotal, all votes will have been cast by 8:00 pm. Virginia polls close at 7:00 pm; Ohio at 7:30 pm, and Florida at 8:00 pm.

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.