Rather than a national election, the US presidential race often appears like a series of contests in less than a dozen states.

That's because both the Republican and Democratic parties rely on consistent partisanship and polarisation in about 80% of US states.

These states have a strong record of voting for a particular party and polling suggests that the trends haven’t changed drastically.

For example, California has voted for the Democratic presidential candidate since 1992 while Texas has voted for the Republican nominee since 1980.

Therefore, candidates see it as unnecessary to campaign in a state that they are almost guaranteed to win or lose.

Efforts instead are diverted to what’s known as swing states/battleground states/purple states - areas which have unclear party loyalties with persuadable voters.

How are swing states defined?

Republican pollster Jon McHenry said swing states are calculated by conducting polling while taking into account margin of error.

"So if a state has 600 respondents and the margin of error is plus or minus 4%, we could say 'well if they’re within the margin on either side, if they’re within say six points… we can say that’s a toss-up state’" he said.

As data can vary from week-to-week, Mr McHenry said assessing historic trends is critical.

"Frankly, if there was a poll that came out tomorrow that said that President Trump was within five points in California, that wouldn't lead me to say that that's a battleground state. That would lead me to say that it's a bad poll" he said.

He added: "Typically, what we're going to do, especially as we start at the beginning of the year, and we don't have a lot of polling to look at, we're going to look at historical performance".

Why are swing states important?

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Swing states are important for their Electoral College votes.

Remember, the US Presidency is not determined by a popular vote, rather what’s called the Electoral College.

You can read an explainer on it here. 

Winning swing states could significantly boost a candidate’s hopes of getting that 270 number.

What it means is that the contest for the presidential election really boils down to a handful of these purple states.

What are the swing states in 2020?

To assess what the swing states are in 2020, we need to look back at the last presidential race as there was a major reconfiguration of the electoral map.

In 2016, all indications pointed towards the usual swing states like Florida, Iowa, Ohio and North Carolina.

But when the results came in, it became apparent that other states were overlooked.

Polls suggested that Pennsylvania, Wisconsin and Michigan were relatively safe blue states but they flipped to red by tiny margins.

In 2016, Donald Trump narrowly won these states along with other swing states like Florida, North Carolina and Arizona.

Winning these purple states along with the reliably Republican voting states, Mr Trump won enough electoral college votes to win the election.

Hillary Clinton also narrowly came out on top in New Hampshire, Minnesota and Nevada.

Therefore, all of these states could be up for grabs in 2020.