US Presidential elections are sprawling affairs, with mail-in voting that begins weeks ahead of Election Day on November 5 and in-person voting that begins at midnight that Tuesday in the small New Hampshire town of Dixville Notch and continues until voting ends at 7 pm in the Hawaiian-Aleutian time zone, already the next day back east. But in the US system, that popular vote doesn’t determine the outcome.

Each state gets a certain number of votes based on its size in what is called the Electoral College. Those votes are awarded on a state-by-state basis, and in all but two states – Maine and Nebraska – the winner takes all. Because many states vote dependably for Democrats or Republicans in presidential elections, the outcome usually rests on a handful of places where the election is truly competitive – the battlegrounds.

This year seven states are regarded as the battlegrounds. They include old-line manufacturing powerhouses that have had a tough transition to the 21st century economy, and fast growing southern states that used to be dependably Republican but are becoming more diverse. All seven have experienced demographic changes that may play into the outcome.

Of the 538 votes in the electoral college, the battleground states will chose 94 of them, or less than 20 per cent.

With the help of narrow wins in Arizona, Nevada and Georgia in particular, President Joe Biden won 302 electoral college votes in 2020.

The battlegrounds have changed over time as the country’s economic and demographic trends have shifted populations around. Floria and Ohio, for example, had been tightly contested in past elections, but now lean Republican in presidential votes. On the other hand shifting demographics have put southern states like North Carolina and Georgia into play for Democrats.

Polls as of early September showed tight races between Vice President Kamala Harris, the Democratic candidate, and former President Donald Trump, running as the Republican nominee for a third straight election.

Voting patterns from past elections show why the battleground states get so much attention when it comes to campaign spending and organisation. Many states are just not that close.

Democrats can bank on the more progressive values among voters in New England and the West Coast to deliver those states to the Democratic candidate, while Republicans have solid support across the more conservative states of the Deep South. In any given election there are few true tossups.

Biden won six of the seven states that were considered battlegrounds in 2020.

Highlight of just seven states

Past voting also shows how the margins have narrowed to make the race competitive in the current crop of battlegrounds.

Trump’s victories in Michigan and Wisconsin in 2016 were particularly surprising among Democrats who counted on their legacy of strong performance in areas with heavy representation among labour unions. Both states flipped to Biden in 2020 and will be central to the 2024 outcome.

While both campaigns will be organising on the ground and tailoring their messages in every battleground state, shifting trends in each may make them more or less receptive to those communications. That is something that Democratic contender Hillary Clinton learned the hard way in 2016 when she failed to understand Trump’s growing strength in Michigan and Wisconsin, where the median age was creeping higher and where core industries had been suffering under a global trade regime Trump campaigned against.

This time around, demographic changes may provide a more receptive backdrop for Harris – at least on the surface. The white population has fallen in all seven battlegrounds, and the Asian, Black and Hispanic populations have mostly increased.

More people also identify in census surveys as belonging to more than one race. Harris herself is the child of Black and Asian parents.

Trump upended expectations in 2016 in part through his appeal to voters who held blue collar jobs and in parts of the country that were more sparsely populated or more economically dependent on the manufacturing industry. He polled worse in areas where higher percentages of the population were college educated. The share of degree holders has risen in all battleground states since Trump won eight years ago, and recent Reuters/IPSOS polling shows Harris with a strong lead among that population.

The number of people of voting age has also been increasing. That has made the competition for younger potential voters fierce in the swing states. In many places the median age is also drifting higher. Trump has tended to poll better among older voters.

Although tens of millions of votes will be cast, the race may well be decided by a few tens of thousands of votes spread across the battlegrounds. The winning margins in those states in 2020 amounted to only about a quarter of one percent of votes cast.

• A Reuters report