US election: What's 270? The maths behind America's electoral system

A woman casts her ballot on the first day of early voting in Overland Park, Kansas. Photo / AP file

It's all about who's going to sit in the White House for the next four years, but to outsiders, American elections can be confusing.

What is the Electoral College? Why do pundits keep talking about 270? Why do the candidates keep hanging out in Michigan and Pennsylvania?

About 96 million votes have already been cast early, 62.3 million by mail and 33.7 million in person. That's about 70 per cent of the total votes in 2016.

Here's a guide to how the United States electoral system works.

IT'S THE ELECTORAL COLLEGE

Nearly 2.9 million more people voted for Democratic candidate Hillary Clinton in the 2016 US presidential election, but she still lost.

President Donald Trump won because he took the Electoral College, under a system set up in the US Constitution and refined through the centuries.

This is where the magic number comes into play. To win the White House, a candidate must win at least 270 electoral votes.

That's a majority of the 538 that are up for grabs in the 50 states.

THE NUMBERS

Each state is allotted a different number of electoral votes, based on how many representatives it has in the House, plus its two senators.

California has the most electoral votes with 55. Texas is next with 38 electoral votes. The candidate who wins New York or Florida can pocket 29 electoral votes towards the race to get 270. Illinois and Pennsylvania each have 20. Rounding out the top 10 list of states with the most electoral votes is Ohio with 18; Georgia and Michigan with 16 and North Carolina with 15.

TRUMP'S BEST PATH

Republican Trump has several paths to 270, but his best route hinges on winning Florida and Pennsylvania. If he wins both states and holds onto North Carolina and Arizona, which he narrowly carried in 2016, and also Georgia and Ohio, which he won in 2016 but is now competitive, he will win. With 29 electoral votes, Florida is arguably the most crucial state for Trump. A loss there would make it nearly impossible for him to retain the White House.

WHAT BIDEN NEEDS

Democrat Joe Biden's campaign is laser-focused on the states in the Midwest and close by that Trump flipped in 2016 — Michigan, Wisconsin and Pennsylvania. He's also making a big play for Arizona, a state that hasn't backed a Democratic presidential candidate since 1996. Biden is also redoubling his focus on Florida, the biggest prize among the perennial battlegrounds and a state that would virtually block Trump's re-election if it swings Democratic. North Carolina and Georgia could be important back-up options.

THE KEY REGIONS

There have been huge voting figures recorded across the Sun Belt in the south west and more moderate voting in the Midwest and neighbouring Rust Belt states. These are the most critical voting regions, alongside Florida in the southeast, which is why the candidates spend so much time there. The two main regions are quite different demographically. For instance Arizona's electorate is 82 per cent urban compared to Pennsylvania's 53 per cent. Hispanics make up 18 per cent in Arizona and 3 per cent in Pennsylvania. Working class voters are at 47 per cent in Pennsylvania and 39 per cent in Arizona.

VOTING PLANS

Polling has shown that more Democrats wanted to make use of mail and early voting options than Republicans who preferred to vote on election day. Those patterns may have changed over the month of early voting. Early voting shows that registered Democrats have returned about 15,000 more ballots in Arizona than registered Republicans. Of 2.4 million ballots sent in early in Pennsylvania, 70 per cent are from registered Democrats and 20 per cent are from registered Republicans.

- AP