Who is ahead in the polls?

Voters in the US go to the polls on 5 November to elect their next president.

The election was initially a rematch of 2020 but it was upended in July when President Joe Biden ended his campaign and endorsed Vice-President Kamala Harris.

The big question now is - will America get its first woman president or a second Donald Trump term?

As election day approaches, we'll be keeping track of the polls and seeing what effect the campaign has on the race for the White House.

Harris saw a bounce in her polling numbers in the first few weeks of her campaign, building a lead of nearly four percentage points towards the end of August.

The polls were relatively stable in September and early October but they have tightened in the last couple of weeks, as shown in the chart below, with trend lines showing the averages and dots for individual poll results for each candidate.

While national polls are a useful guide as to how popular a candidate is across the whole country, they're not the best way to predict the election result.

That's because the US uses an electoral college system, in which each state is given a number of votes roughly in line with the size of its population. A total of 538 electoral college votes are up for grabs, so a candidate needs to hit 270 to win.

There are 50 states in the US but because most of them nearly always vote for the same party, in reality there are just a handful where both candidates stand a chance of winning. These are the places where the election will be won and lost and are known as battleground states or swing states.

• What is the electoral college?

Right now the leads in the swing states are so small that it's impossible to know who is really ahead from looking at the polling averages.

Polls are designed to broadly explain how the public feels about a candidate or an issue, not predict the result of an election by less than a percentage point so it's important to keep that in mind when looking at the numbers below.

It's also important to remember that the individual polls used to create these averages have a margin of error of around three to four percentage points, so either candidate could be doing better or worse than the numbers currently suggest.

If you look at the trends since Harris joined the race, it does highlight some differences between the states.

In Arizona, Georgia, Nevada and North Carolina, the lead has changed hands a few times since the start of August but Trump has a small lead in all of them at the moment.

In the three other states - Michigan, Pennsylvania and Wisconsin - Harris had led since the start of August, sometimes by two or three points, but the polls have tightened significantly and Trump now has a very small lead in Pennsylvania.

All three of those states had been Democratic strongholds before Trump turned them red on his path to winning the presidency in 2016. Biden retook them in 2020 and if Harris can do the same then she will be on course to win the election.

In a sign of how the race has changed since Harris became the Democratic nominee, on the day that Biden quit the race he was trailing Trump by nearly five percentage points on average in the seven swing states.

In Pennsylvania, Biden was behind by nearly 4.5 percentage points when he dropped out, as the chart below shows. It is a key state for both campaigns as it has the highest number of electoral votes of the seven and therefore winning it makes it easier to reach the 270 votes needed.

The figures we have used in the graphics above are averages created by polling analysis website 538, which is part of American news network ABC News. To create them, 538 collects the data from individual polls carried out both nationally and in battleground states by lots of polling companies.

As part of its quality control, 538 only includes polls from companies that meet certain criteria, like being transparent about how many people they polled, when the poll was carried out and how the poll was conducted (telephone calls, text message, online, etc).

You can read more about the 538 methodology here.

The polls have underestimated support for Trump in the last two elections and the national polling error in 2020 was the highest in 40 years according to a post-mortem by polling experts - so there’s good reason to be cautious about them going into this year’s election.

The polling miss in 2016 was put down to voters changing their minds in the final days of the campaign and because college-educated voters - who were more likely to support Hillary Clinton - had been over-represented in polling samples.

In 2020, the experts pointed to problems with getting Trump supporters to take part in polls, but said it was “impossible” to know exactly what had caused the polling error, especially as the election was held during a pandemic and had a record turnout.

Pollsters have made lots of changes since then and the polling industry “had one of its most successful election cycles in US history” in the 2022 midterm elections, according to analysts at 538.

But Donald Trump wasn’t on the ballot in the midterms and we won’t know until after election day whether these changes can deal with the influx of irregular voters he tends to attract.

• Listen: How do election polls work?