Today Americans will choose between Democrat Kamala Harris and Republican Donald Trump in the US presidential election.

This closely watched election around the world also includes the election of members of Congress who will play a critical role in shaping U.S. laws.

The winner's term is four years, starting in January 2025.

The candidates

In the Republican Party, former President Donald Trump emerged as the nominee with strong support over his competitors. He chose Ohio Senator JD Vance as his running mate.

Kamala Harris, the current vice president, stepped in as the Democratic nominee after President Joe Biden decided not to run again. Her vice president is Minnesota Governor Tim Walz.

There are also independent candidates, including Robert F. Kennedy Jr., although he withdrew in August and endorsed Trump.

This is how the election works

The US uses an electoral college system instead of a direct popular vote, as described on the US government's official website, USA.gov.

This means that the candidate with the most national votes is not necessarily the winner.

Each US state is allocated a certain number of electoral votes based on its representation in Congress.

This apportionment consists of two votes for each state's senators and additional votes equal to the number of congressional districts within that state.

In the US, there are 50 states and 538 votes in the Electoral College, so a majority of 270 votes is required to win.

States like Alaska, Vermont and Wyoming only have three electors, while Florida has 30. California, the most populous state, has the highest number of votes with 54.

The winner is not the person who receives the most votes nationwide. Instead, both candidates are competing to win contests in the 50 states.

In most states, the winner-take-all system applies: the candidate with the most votes in a state wins all the electoral votes.

Only Maine and Nebraska allow their electoral votes to be apportioned based on results in individual congressional districts.

Their distribution is based on the popular vote within the state and at the congressional district level.

Battleground states

While many states vote fairly consistently for either Democrats or Republicans, some states, known as “swing” or “battleground” states, can vote either way and often determine the outcome of the election.

Candidates are ramping up their campaign efforts in these key battleground states, focusing on winning over undecided voters.

CBS News considers these states to be the battlegrounds for the 2024 presidential election: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

A candidate can lose the national popular vote but still win through the Electoral College, as was the case in 2016 when Trump won even though Hillary Clinton received more votes overall.

Only five U.S. presidents have been elected without winning the popular vote, three of them in the 19th century.

Unusual scenarios

In rare cases, members of the Electoral College, called “electors,” cast their votes for someone other than the candidate who won their state.

They are called “faithless electors.”

If neither candidate reaches 270 electoral votes, resulting in a 269-269 tie, the decision goes to the House of Representatives in a so-called “contingent election.”

AFP reports that the last time this happened was in 1800, when Thomas Jefferson finally won.

If a tie requires a vote in the House of Representatives, each state delegation in the House of Representatives receives one vote and the candidate with the majority of state votes becomes president.

The Senate would then elect the vice president, with each senator casting one vote.

This process could result in a president and vice president from opposing parties.

This means that it is theoretically possible that Trump will be elected president with Democratic vice presidential nominee Tim Walz, or that Harris will be elected president with Republican vice presidential nominee JD Vance.

If such a vote in the House of Representatives becomes necessary, it will take place on January 6, 2025.