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When Donald Trump won the 2016 US election it was one of the biggest upsets in American political history, but will he win again in 2020?

The US election happens every four years on the first Tuesday of November, with this year's election happening on 3 November.

Mr Trump is hoping to win a second term and another four years in the White House. His rival is Democratic Party candidate Joe Biden, who was vice president to President Barack Obama from 2009 to 2017.

Coronavirus has meant a record number of people aren't voting in person this year and are instead making their decision using postal votes. This could mean the result may not be declared on election night, but may take several days - or even weeks - before it's announced.

Despite any delay to the result, the man who wins will be inaugurated in January 2021 and will have one of the most powerful jobs in the world.

But which voters, and more importantly which US states, will decide the 2020 US election?

In America more than 245 million adults are eligible to vote - but in the US election, the number of votes you win is less important than where you win them.

That's thanks to something called the Electoral College - which is basically a winner takes all contest in each state.

The word 'college' in Electoral College refers to a group of people known as electors.

Each US state has been given a number of electors based roughly on the number of people who live there. The more people living there, the more electors a state has.

In most states, the presidential candidate who wins the most votes gets the support of all of that state's electors. Even if they win by the smallest of margins.

So for example, the Democrats are likely to win in California. That state has lots of people living there, so Joe Biden would get all of California's 55 Electoral College votes.

But a win in Vermont, which has fewer people living there, means just three electoral votes.

Added together across all 50 states and the capital, Washington DC, there are 538 votes in total.

The first candidate to win enough states to reach 270 electoral votes becomes the president.

It was this system that saw Donald Trump beat Hillary Clinton to become president in 2016.

Despite 2.87 million more people voting for Mrs Clinton across the country, Mr Trump won in the important states, meaning he received 304 electoral college votes to Mrs Clinton's 227.

This means that presidential campaigns often focus on key states to try and win the election.

Most of the US states usually vote the same way in each election, but there are a few states where both candidates have a chance of winning in 2020.

Although some are more likely to switch sides than others, these are the places where the election will be won and lost, and are known as battleground states.

For example, the Republican party tends to win in Texas, which would be good news for Donald Trump because it has the second most electoral votes in the country, with 38.

A win for the Republicans in Texas is likely, but it's not guaranteed and if Mr Trump were to lose there, it would be pretty significant for the overall outcome of the presidential race.

Elsewhere Florida, which has 29 electoral votes, is important because it's known as a swing state. Basically that means Florida often 'swings' from one party to another during presidential elections.

In the last six elections, Florida has voted for the Democratic party three times and the Republican party three times. So it really could go either way this year.

Ritu Parsad is a ³ÉÈË¿ìÊÖ North America journalist working in Florida, she said:

"Presidential candidates always put a lot of effort into speaking to people here, as their votes are really valuable.

"Swing states are where the election is won and lost and where parties focus their work. That's why Donald Trump is visiting Florida several times ahead of the election and why one Democratic donor gave 100 million dollars to help Joe Biden's campaign here," she explained.

Other important states to look out for are North Carolina, Michigan, Pennsylvania, Wisconsin, and Arizona. All could be crucial for the candidate wanting to be sat in the White House come January.

They are all states where Donald Trump won narrowly against Hillary Clinton in 2016 and could swing the other way this year.

Four years ago Donald Trump also had big wins in the swing states of Iowa and Ohio, where his winning margin was between 8 and 10%, but election polls suggest things will be much, much closer this time.

Dr Clodagh Harrington is an Associate Professor in American Politics at De Montfort University in Leicester, speaking to Newsround she said that while you can 'trust the polls' they are 'not votes'.

"A person can be polled, say certain things and then change their mind or choose not to vote at all," she said.

"You can trust the polls reasonably well, but polls are not votes."

The 2016 election proved just how important every vote in swing states can be.

When Trump won in places such as Pennsylvania, Michigan and Wisconsin four years ago, he did so with a combined total of 107,000 votes.

That's just 0.03% of the entire population of the United States.

Dr Harrington says results in states such as Wisconsin could be crucial again this year:

"On important issues, such as racial justice, both candidates have a strong message, but are speaking a different language," she said.

"Following the [Black Lives Matter] protests, Trump had the message of law and order, while Biden has talked about changing the way the police behave."

In a close race, the 2020 result could come down to which candidate can convince a swing voter to switch sides because of key issues, or persuade a reluctant voter to make a choice; Trump or Biden.