Millions of Americans will take to polling places across the country on Tuesday with the future of the American presidency and Congress on the line.

Vice President Kamala Harris and Republican nominee Donald Trump are locked in a virtual dead heat nationally, with most polling showing them narrowly split by between 1% and 3%, well within the margin of error on most of the surveys.

A total of five polls have Trump and Harris locked in a virtual tie.

The race somehow gets even tighter when looking at the seven key swing states, where an average of polling from four of the states – Nevada, Wisconsin, Michigan and Pennsylvania – have the nominees separated by 1% or less.

Trump, however, has a relatively larger lead in Arizona, Georgia and North Carolina, but even there he is leading by an average of less than 3%.

Battleground states are pivotal because unlike most modern democracies the US does not directly elect its presidents. Instead, the process plays out via the Electoral College, where 538 representatives cast their ballots in line with their states’ outcomes.

Either candidate needs to secure 270 Electoral College votes to claim victory. Electors are allocated to states based on their population, and most states give all of their electors to whichever candidate wins the state in the general vote.

The winner-take-all model is not followed in Nebraska and Maine, however, which instead allocate their votes proportionally.

Further down the ballot, voters will determine the composition of the next US Congress.

In the Senate, 34 seats are up for election. Senators are elected to six-year terms, and one-third are elected every two years. Roughly four of the races are considered toss-ups, including contests in Michigan, Ohio, Pennsylvania, and Wisconsin that are currently held by Democrats.

Republicans are slightly favored to win numerical control of the Senate, but whoever emerges victorious will be left to navigate a precarious razor-thin majority. In the 100-seat Senate, due to procedural rules, parties often need 60 rather than just 50 votes to pass legislation.

All 435 seats in the House of Representatives are up for election, and as with the Senate, most forecasts have the chamber near-evenly split. Just a couple of dozen competitive elections will determine whether Republicans or Democrats will control the House.

At the state and local levels, voters will decide on a range of initiatives and races from school boards to state-level ballot measures that can hold the weight of law. A total of 11 governor’s races are being contested across the nation.