US presidential races: Close-run campaigns
The year 2012 may be remembered as one of the tightest US presidential election races in history, with Democrat President Barack Obama and his Republican challenger Mitt Romney neck-and-neck as campaigning closes.
In the US, the winning candidate does not need to win the national popular vote. To become president, a candidate needs 270 of the 538 Electoral College votes apportioned to each state according to how many House of Representatives members and senators they have in the US Congress.
In a close contest, more attention focuses on the so-called swing states, the handful of battlegrounds that do not reliably vote either Republican or Democrat in each election.
Here, we look at some other nail-biting races over the past century.
1916 : Woodrow Wilson and Charles Hughes
As World War I raged in Europe, Democratic President Woodrow Wilson campaigned with the slogan "He kept us out of war". His pledge to remain neutral was extremely popular among Americans.
Pitted against the Republican candidate Supreme Court Justice Charles Evans Hughes, he won an incredibly tight race in 1916.
The race ultimately came down to the state of California where Hughes made what some historians say was the error of not meeting with the powerful governor, who subsequently withheld his full support.
Wilson secured a second term with just 49.2% of the popular vote and 277 Electoral College votes.
1960 : John F Kennedy and Richard Nixon
The race between the Republican Richard Nixon and the younger, Catholic Democrat John F Kennedy was one of the closest in history.
Nixon - vice-president to the retiring President Dwight Eisenhower - campaigned in all 50 states to win the White House. The race was competitive in 20 states where the margin of victory for either candidate was narrower than five percentage points.
Some say a turning point in the race was the first live televised debate between the two, where the younger man appeared confident and charismatic while Nixon was shown wiping beads of sweat from his brow.
In the event, Kennedy scraped victory with 49.7% of the vote compared with Nixon's 49.6% - a mere 113,000 votes separating the two men in the popular vote of 68 million cast. But the electoral college margin was wider - 303 to 219.
Republicans - though not Nixon himself - pushed for a recount in a number of close states amid a flurry of rumours circulating about fraud.
1976 : Jimmy Carter and Gerald Ford
Republican Vice-President Gerald Ford took office after Nixon resigned in the wake of the Watergate scandal. But Ford's decision to pardon Nixon was unpopular.
His opponent was former state senator and Georgia Governor Jimmy Carter, so little known on the national stage that he introduced himself with the line: "Hello! I am Jimmy Carter, and I am running for president."
Carter, a former nuclear engineer and farmer, ran as a political outsider, untainted by Washington at a time when trust in politicians was at an all-time low.
The pair agreed to a televised debate, the first since Nixon and Kennedy's in 1960. Ford's statement in the second debate that the Soviet Union did not dominate Eastern Europe - and never would in a Ford administration - made some voters doubt his grasp of international affairs.
In the end, Carter won with 50.1% percent of the popular vote compared to Ford's 48% and an electoral college margin of 297-240. The 27 states that Ford won remain the most ever carried by a losing candidate.
2000 : George W Bush and Al Gore
It was the closest vote in US history - and one of the most controversial - pitting the Vice-President and Democrat Al Gore against the governor of Texas and son of a former US president, George W Bush.
Gore won 48.38% of the nationwide total vote to Bush's 47.87%.
But, after the US Supreme Court halted a recount in the state of Florida, Bush had won the state vote by the slimmest of victories - just 537 ballots of some six million cast - and with it the Sunshine State's decisive 25 electoral college votes, which gave him a winning total of 271.
2004 : George W Bush and John Kerry
As incumbent in 2004, George W Bush faced Massachusetts Democratic Senator John Kerry.
Just months into Bush's first term, the attacks of 11 September 2001 had struck the US. He campaigned in 2004 on national security, presenting himself as a decisive leader and dismissing Kerry as a "flip-flopper".
Days before the election, excerpts of a message from al-Qaeda leader Osama Bin Laden were broadcast in which he claimed responsibility for the attacks and taunted Bush for his subsequent decisions.
After the broadcast, Bush's lead strengthened and in the end he took 286 electoral votes compared to 251 for Kerry.