Polling stations have opened in numerous US states to decide not only who will become the country's next president, but also the composition of the House of Representatives and the Senate.

Election Day on Tuesday is the culmination of weeks of early voting in some places. Several states began early voting — whether by mail or in person — back in September.

According to the University of Florida's Election Lab, nearly 81 million voters have already cast their ballots before Election Day.

Democratic candidate Kamala Harris and Republican rival Donald Trump are in a neck-and-neck race that remains too close to decide.

Whatever the outcome, the outcome will determine U.S. policy and policy for the next four years. It will also be historic because voters will elect either Harris, the first female president, or Trump, the first convicted felon.

The vote will also ultimately reveal the full extent of the changing demographics of the US electorate.

The first voting site technically opened just after midnight Monday Eastern Time (Tuesday, 5 a.m. GMT) in the small New Hampshire town of Dixville Notch.

Once the polls close in the evening, it can take hours or days for the results to be tabulated. States cannot begin reporting their vote counts until polls close.