The positions of president and vice president, currently occupied by Bush and Dick Cheney, are up for election every four years.
In addition, the entire House of Representatives and one-third of the Senate – the two parts of Congress - are elected simultaneously with the president. And 11 states are going to choose a governor.
Before then, voters in each state have a chance to pick a challenger to the incumbent. Aljazeera's guide to the 2004 election campaign explains who has their eyes on the White House.
Who wants to be president?
President Bush is almost certainly going to be his Republican's party's choice for the top job and Cheney is likely to remain his deputy. Some nationally unknown Republican candidates are theoretically in the race, but stand little chance of making any impression.
Although Bush enjoyed a strong lead in the opinion polls for more than two years going into 2004, he is taking no chances and has raised more than $140 million to ensure his re-election.
The Democrat party has not nominated its challengers yet – eight significant candidates were vying for selection as the 2004 campaign began.
Who are the Democrat hopefuls?
The candidates with the likeliest prospects at the start of the campaign were Wesley Clark, a retired general and former NATO commander; Howard Dean, the former governor of Vermont; John Edwards, senator for North Carolina; Richard Gephardt, a representative for Missouri and former House minority leader; John Kerry, senator for Massachusetts; Joseph Lieberman, senator for Connecticut.
Beaten candidate in 2000 Al Gore
(L) has endorsed Howard Dean
Dennis Kucinich, a leftwing representative from Ohio, and Al Sharpton, a civil rights activist also joined the running, but have been seen as fringe candidates.
Dean was the front runner heading into 2004, with Clark, Lieberman and Gephardt closest behind him in national opinion polls. But Kerry overtook them all once the primary and caucus elections began.
Weren't there others?
Carol Moseley Braun, the former senator for Illinois and the first African American woman in the Senate, dropped out in mid-January. Bob Graham, a conservative senator from Florida, quit in October 2003.
There are several other registered Democrats hoping to win their party's nomination, but stand little chance.
One such is Warren Ashe of Virginia, who claims to be "President, United Nations, 1973-2003" and "Appointed President, United States White House, 1981-2003". He also says he owns a $40 million business and was in the navy, army and air force too. Hmm.
Stressing individual liberty, the third biggest party is the Libertarian party, which in the 2002 elections won more than a million votes - more than twice those received by all other minor parties combined. Among their candidates is Talk radio host Gary Nolan.
The pro-environment Green party is fielding a handful of candidates.
Ralph Nader was the Greens' nominee in 1996 and 2000, but this time is standing as an independent.
There is also the rightwing Constitution party, which believes that US law is based on Biblical law and wants to abolish federal income tax.
But the bravest (and most optimistic) candidates are probably those from the two socialist parties.