In order to become president, either candidate needs to win a majority of the 538 electors – so 270.
In July 2020, the US Supreme Court ruled that electors must follow the popular vote in states that have passed such a law.
Critics have questioned the electoral college system which does not guarantee the candidate with the most votes gets to the White House.
For example, in 2016 Hillary Clinton won 48.2 per cent of the vote and Donald Trump 46.1 per cent. But as he won more states, he was elected president.
How do parties choose their candidate?
The first stage of the 2024 elections will see the two main political parties select their presidential candidates.
Some states go to the polls in primary elections to choose delegates for the party convention. Other states rely on the caucus system where party members gather in precincts and debate who should be the candidate before choosing delegates accordingly.
There are differences between the parties. The Republicans hold a secret ballot in caucuses, while the Democrats form groups and hold a succession of votes, with candidates who get less than 15 per cent of votes dropping out, and their supporters picking an alternative.
While states pay for primary elections, the parties pick up the bill for caucus meetings.
Delegates then gather at their national convention to nominate the presidential candidates. In recent elections, this has been a formality. The last contested convention was in 1976 when amid chaotic scenes when Gerald Ford fought off a challenge from Ronald Reagan.
What is Super Tuesday?
“Super Tuesday” is when 15 states hold primaries or caucuses to select their 2024 candidates.
It is often make or break for candidates. In 2020 it helped Joe Biden take a massive step towards the nomination after Elizabeth Warren split the Left-wing vote, much to the fury of supporters of Bernie Sanders.
What happens on election day?
The majority of voters go to the polls and ballots are counted. Many people may have already cast their vote ahead of time using the postal voting system or early voting.
Votes are counted in each state after their respective polls close. Poll close times vary from state to state. The winner of the election will likely not be projected for days. Even then, the results are not finalised properly for months.
What are the key battleground states?
Focus will be on the states where the results were challenged by Mr Trump in the 2020 election.
Experts have predicted that Arizona, Georgia, Wisconsin and Nevada will be the key four states. However, nothing is set in stone.
Florida and Ohio, once considered marginal, are now safe Republican territory.
What happened in 2020?
In 2020 Mr Trump refused to accept defeat and contested the election results in Pennsylvania, Michigan, Arizona, Nevada and Georgia.
His challenges were repeatedly dismissed out of hand by the courts, paving the way for the storming of the Capitol on January 6 by his supporters.