The US election campaign is in its final weeks with voters heading to the polls on November 5 to elect the next president.

Early voting is already taking place in a number of states, including in battlegrounds such as North Carolina and Georgia.

With less than three weeks to go until the election, Democratic Vice President Kamala Harris and Republican former President Donald Trump are battling it out to sway undecided voters.

However, the popular vote does not decide the winner. Instead, it determines which electors will represent each state in the Electoral College, which does pick the president.

To win, a candidate needs to secure 270 of the 538 electoral votes up for grabs. Electoral College votes are distributed across states according to their relative populations.

Who is in the lead?

According to FiveThirtyEight’s daily election poll tracker, Harris is currently leading in the national polls and has a 2.4-percentage-point lead over Trump.

In July, President Joe Biden, a Democrat, exited the presidential race and endorsed Harris as his replacement. Since then, the vice president’s ratings have risen from what had been lower numbers under Biden.

But the race is still tight. FiveThirtyEight’s election forecast suggests that Harris is favoured to win 54 times out of 100 while Trump wins 46 times out of 100.

Which states could swing the presidential election?

Swing states, also known as battleground states, can sway the outcome of a national election.

One of the defining characteristics of a swing state is its ambiguous political leanings where no party has overwhelming support.

This year, the states being closely watched are Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

In the 2020 presidential election, Georgia flipped from Republican red to Democratic blue after nearly three decades of voting Republican, and in Arizona, the Democrats won by a narrow margin of 0.3 percentage points.

Polls show Trump and Harris in a narrow race in swing states. Polling in these states is more important in determining the winner than national polling because the Electoral College and not the popular vote chooses the president.

What happens if Harris and Trump end in a tie?

There are a total of 538 electoral votes. To win the election, a candidate must get 270.

Given the nature of how electoral votes are distributed, specific combinations of states could lead to a tie of 269 votes. Such a scenario is possible, though not probable.

If no candidate wins at least 270 electoral votes, a contingent election takes place in which the US House of Representatives decides the winner.

Each state’s delegation in the House would cast one vote, and a candidate must receive a majority (26 out of 50) of the state delegation votes to win.

The US Senate would then choose the vice president with each senator casting one vote and a simple majority (51 votes) required to win.

How do polls work?

Election polls predict how the population might vote by surveying a sample of voters. Surveys are most commonly conducted by phone or online. In some cases, it is via post or in person.

Poll trackers, which aggregate a number of polls together, are weighted based on a number of factors, such as the sample size of the poll, the pollster quality, how recently the poll was conducted and the particular methodologies employed.

How accurate are polls?

Polls are never 100 percent accurate. Both the 2016 and 2020 US elections saw opinion polls underestimate the popularity of Republican candidates. Despite polling for the 2022 midterms being more accurate, many still remain sceptical about polling results.

Part of the reason for inaccuracy in polls in recent years is related to the ability to reach voters. Often polls are conducted via telephone surveys; however, fewer people are inclined to answer calls. Nonresponse bias is another reason for inaccuracy – for example, in recent years, Trump voters have chosen not to respond to polls. Additionally, changes in voter turnout have affected the accuracy of polls, for example, in 2020 voter turnout was much higher than expected.

A mathematical margin of error is implicit in polls because they use small, select groups of people to ascertain the choice of a larger population. That margin of error in US polls indicates the range within which the actual result is likely to fall. With a 1,000-person sample size, the margin of error is about plus or minus 3 percent.

Many of the polls conducted before this year’s presidential election have shown the difference in support between Harris and Trump within the margin of error.