In US politics, there is usually an incumbent running for the White House - if not the president himself (and so far it has always been a "himself"), then his vice-president.
But with George W Bush constitutionally limited to two terms and VP Dick Cheney opting not to run, the race is wide open for the first time since 1928.
George W Bush will leave the White House after the two-term limit
Election Day - 4 November 2008 - may still feel a long way off, but the primary elections, in which each party chooses its nominee, have already begun.
The lack of an incumbent made for an unusually crowded field to begin with, but after battling to raise money in 2007, and poor performances in the early primaries in 2008, a number of candidates have now dropped out of the race.
Here are the candidates who are still in the contest:
Who is she? The first former First Lady to go on to hold elected office, she is now serving as senator for New York. Declared her intention to stand with a video saying "I'm in to win" on her website on 20 January 2007 - two years to the day before the next president is inaugurated.
Why take her seriously? Unbeatable name recognition and serious fundraising ability made Hillary - no surname necessary - an early front-runner for the Democrats. She has tried to stake out a position as a centrist in her six years in the Senate. She reported $26m in fundraising from the first quarter of 2007 (plus $10m from her 2006 Senate campaign fund) and netted a further $54m in the next two quarters. After a surprise third-place finish in the Iowa caucus, her equally surprising win in the New Hampshire primaries demonstrated that she should never be dismissed.
What is going to stand in her way? If Bill Clinton remains a divisive figure in American politics, that goes at least double for Hillary - some estimates say one in three Americans would never vote for her. And the US has never yet elected a woman president. Her fundraising efforts have not been enough to shake off close rivals, notably Barack Obama, who managed to defeat her in Iowa.
Did you know? Is there anything we do not already know about Hillary Clinton?
Who is he? Mixed-race junior senator from Illinois who shot to prominence at the 2004 Democratic National Convention. He began his official campaign on 10 February 2007 with a call for the Iraq war to end, saying US troops must withdraw by March 2008.
Why take him seriously? Time magazine dubbed Senator Obama "America's hottest political phenomenon", while television's own phenomenon, Oprah Winfrey, urged him to announce his candidacy on her show. He impressed observers by raising $25m in funds in the first quarter of 2007 - only $1m short of Hillary Clinton's total - and overtook her in the second quarter with a record $32.8m. With three months to go before the primary season, he had $32m to spend on campaigning, compared with her $35m. His win in the Iowa caucus - the first test of the electoral calendar - electrified the Democratic race.
What is going to stand in his way? Barack Obama will have served only four years in the Senate when the 2008 election rolls around - even less than John F Kennedy did before he was elected in 1960. And while his supporters praise him as thoughtful, deliberative and liberal, some experts doubt those are the characteristics that get a politician to the White House.
Did you know? Barack Obama's mother was from Kansas and his father was from Kenya - leading observers to suggest he is an African and an American, but not an African-American.
Who is he? Governor of Arkansas from 1996 to 2006, best known nationally for his dramatic weight loss in response to a diagnosis of diabetes. Also named to Time magazine's 2006 list of the nation's best governors.
Why take him seriously? Gov Huckabee has carved out a place for himself as a health advocate following his own turnaround from fat to fit. As an ordained Baptist minister who says his faith is inseparable from his politics, he has been a magnet for conservative Christian votes - especially in comparison to the other leading Republicans. He won an important victory in the Iowa caucus and is doing well in national polls.
What is going to stand in his way? His Evangelical faith has led him to take positions that do not appeal to all Republicans, such as an increase in the state's minimum wage - which drew fire from fiscal conservatives. He is also lagging behind better-known names when it comes to fundraising and campaign organisation.
Did you know? Gov Huckabee plays bass guitar in a band called Capitol Offense.
Who is he? Maverick senator from Arizona who ran against George W Bush for the Republican nomination in 2000 and has had complex relations with the president ever since. He officially launched his candidacy in April 2007.
Why take him seriously? John McCain is one of the highest-profile figures in Washington, known for sponsoring key measures against torture and in favour of campaign finance reform. Despite falling behind the front-runners in many polls in 2007, he managed to turn round his campaign and win the New Hampshire primary by a large margin.
What is going to stand in his way? Although Senator McCain says unequivocally that he is a social conservative, his frequent skirmishes with the Bush administration have made him more popular with liberals than with some conservatives. He has also doggedly backed America's keeping troops in Iraq, an increasingly unpopular position. He overhauled his fundraising operation after a disappointing first quarter of 2007 but this was not enough to turn the tide, prompting a major shake-up of his operation in summer 2007.
Did you know? Shot down as a Navy pilot over Vietnam in 1967, Mr McCain refused to be released ahead of other prisoners of war - earning himself five years of captivity, two in solitary confinement.
OTHER OFFICIAL CANDIDATES
Former senator from Alaska
Representative from Texas