US 2020 presidential election: How does it work?
By ITV News Westminster Producer Lewis Denison
Donald Trump's fierce campaign for re-election is underway, and Democratic candidates are racing to be selected to face him in the election.
The caucuses have started and Mr Trump has began releasing attack ads.
Election day isn't till November 3, but Mr Trump's campaign began a whole 16 months before voters get to make their choice.
In the UK prime ministerial hopefuls officially have just 38 days to campaign before an election, so why is America's presidential election cycle so long?
Here's a detailed breakdown:
Stage one: Candidates announce their intentions to run
Even before an incumbent president - this time Donald Trump - officially launches their re-election campaign, candidates from the opposing wing of politics announce their intentions to run - this time the Democrats from the left.
As opposed to the UK system, where a party leader is already selected to lead the fight for election, in America candidates are selected through a series of primaries and caucuses and anyone can throw their hat into the ring - so long as they are able to tick three boxes:
1. A natural born US citizen2. Over 35 years of age3. US resident for over 14 years
Due to there being so few requirements to become president, the field of candidates is often densely packed - 23 Democrats announced their intentions, however several have already dropped out.
In every election both new and old faces announce their intentions to run and this year is no different.
Recognisable politicians hoping to win the Democratic nomination include former vice president Joe Biden and 2016's outsider Bernie Sanders.
A new face some won't recognise is Beto O'Rourke, a rising star in the Democratic Party who failed to win a senate seat in Texas last year.
He dropped out the race in November 2019 but he is likely to try again in the future.
Candidates from the incumbent party are also able to announce their intentions to run but never in American history has an incumbent president lost in the primaries - but Gerald Ford came very close to losing to Ronald Reagan in 1976.
Stage two: Primary and caucus debates
Throughout the summer, in the year before an election, a series of debates take place in several key states across America.
This time the Democrats have so far scheduled six debates which are intended to help voters decide who they want to nominate.
The first debates took place in Miami, Florida, the second in Detroit, Michigan and more are still yet to happen.
After the first rounds of debate the candidates left standing (some drop out before the next stage due to lack of support) head to the primaries and caucuses where they hope to receive their party's nomination.
Stage three: The primaries and caucuses
Primaries and caucuses take up the bulk of the presidential election cycle - they run from the start of February in election year and finish with the parties' national conventions at the end of the summer.
Primaries and caucuses are run differently but both serve the same purpose of selecting candidates for nomination - they function as preliminary elections.
In an election cycle every one of the 50 states holds a primary or caucus in one form or another however not all candidates will attend every event.
The main difference is that primaries use secret ballots for voting whereas caucuses are local gatherings of voters who vote at the end of the meeting for a particular candidate.
Famously, the first primary is always in New Hampshire and the first caucus is always in Iowa.
Another well known occasion in the primary stage of the electoral cycle is Super Tuesday, where 13 states hold primaries and caucuses.
Those states are Alabama, Arkansas, California, Colorado, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah, Vermont, and Virginia.
The purpose of every primary and caucus is to select delegates who, on behalf of their particular state, will select the party nominee at the national conventions.
Sanders calls for political revolution as he launches 2020 campaign
Donald Trump predicts he'll be re-elected with increased majority
Warren calls for change as she makes presidential bid official
Stage four: The national conventions
In most cases the party nominee is already clear by the time of the national conventions and they're often a place for the winning candidate to celebrate.
Those not selected as nominees often use the national conventions as a chance to congratulate the winner.
The convention is usually where the party nominee will announce their vice presidential running mate.
In rare cases where primaries and caucuses have not been able to determine a winning candidate, the nominee will be selected at the convention by the delegates.
There are two kinds of delegates with different functions at the conventions: Pledged, or bound delegates or unpledged, unbound delegates or superdelegates.
Bound delegates are required to support the candidate they were awarded to through the primary or caucus process.
Superdelegates can, regardless of which way their state voted, support any presidential candidate they choose.
If no candidate has a majority at the convention, several rounds of voting may take place:
In round one of voting the bound delegates usually vote for the candidate they were awarded to at the start of the convention
In the first round of voting, pledged delegates usually have to vote for the candidate they were awarded to at the start of the convention but unpledged delegates don't vote.
If no nominee wins in the first round, the pledged delegates may choose any candidate in later rounds of voting and balloting continues until one candidate receives a majority.
Once both parties have chosen their nominee the real race begins with general election campaigning.
President Trump's 2020 campaign begins as voters give their view on first term
Donald Trump marks four years since starting his journey to the Oval Office
Stage five: General election campaigning and presidential debates
Once the nominees have been selected they will travel the country as they aim to appeal to voters at rallies and town hall meetings.
They will explain their views, plans and policies as they aim to secure the presidency.
This is the part of the race where most of the famously-aggressive attack adverts will be released.
While at this stage attack ads become more vicious it is likely lots have already been released before this point.
Another way the party nominees can edge in front of their rivals is in the head-to-head presidential debates which take place in September and October.
Presidential debates are huge events in America and they're often where elections can be won and lost.
The campaign ends with election day, however, this is not the race's finish line because after that the sometimes controversial electoral college steps in.
Stage six: Election day and electoral college
As is the case in the UK, voters head out to polling stations on election day and instead of directly electing the president, votes go toward statewide tallies.
The tally for each state translates into a number of electoral college votes which are cast by electors.
In 48 states plus Washington DC the winner gets all the electoral college electors but in Maine and Nebraska they're assigned using a proportional system.
Election day is on November 3 but the electoral college is held in mid-December.
The winner of the election is usually announced the morning following election day once the votes are counted however it is possible to win the popular vote and lose the election.
This happens when a nominee gets less overall votes than their opponent but wins more electoral college votes.
It could be the case that the losing candidate won lots of small states with huge majorities and so only gained a small number of electoral college electors but the winning candidate may have narrowly won lots of large states and as a result achieved a high number of electors.
Once the winner is finally decided the vice-president-elect and president elect wait for their celebration when they're sworn in on inauguration day.
Stage seven: Inauguration day
Inauguration day takes place every four years on January 20th, unless that day is a Sunday, in which case the event takes place on the 21st - it's a national holiday in America.
It is the day thousands of spectators turn out to watch both the vice president and president as they are sworn into office at the Capitol building in Washington, DC.
The vice president is sworn in first, and repeats the same oath of office which has been used by senators, representatives, and other federal employees since 1884.
He or she says: "I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter: So help me God."
It is the president's turn next and he or she gives their oath of office around noon: "I do solemnly swear (or affirm) that I will faithfully execute the office of president of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."