WASHINGTON (REUTERS) - The coronavirus pandemic, an unprecedented number of ballots cast early, the lack of consistency about how these votes will be counted, and ongoing legal battles have made the outcome of the 2020 US presidential election one of the hardest to predict.

As Americans head to polling stations on Tuesday (Nov 3), the question is not just whether Republican President Donald Trump will win a second four-year term or be defeated by his Democratic rival Joe Biden, but also when the result will be known.

The latest opinion polls show the race is close enough in the battleground states to swing the outcome to either party, even as Mr Biden leads Mr Trump in national polls.

Some of these states do not begin to count early votes until after polling stations close, and some allow ballots that arrive after election day to be included as long as they are postmarked by Nov 3.

If the presidential race depends on the outcomes in these states, America could be waiting for days.

Experts have cautioned against reading too much into early returns, which could be distorted by how each state processes the votes not cast in-person on election day.

Here are some moments to look for on Tuesday and beyond:

NOV 3

5pm ET (6am Singapore time on Wednesday): Edison Research will release preliminary findings from its exit polls, which are based on in-person interviews with voters on election day, in-person interviews at early voting centres before Nov 3, and telephone interviews with people who voted by mail.

The initial data will look at national and state voter sentiment and motivations, but not detailed percentage estimates.

Results from ballot questions in individual states will be released after voting ends in the state.

Edison will refine and update its national and state exit poll results through the night, gathering more voter responses and adjusting the weightings to reflect turnout.

6pm ET (7am Singapore time on Wednesday): Some polling stations begin to close in the Republican strongholds of Indiana and Kentucky, the first in the country to close.

7pm ET (8am Singapore time on Wednesday): Voting ends in Georgia, Indiana, Kentucky, South Carolina, Virginia and Vermont.

Some polling stations begin to close in Florida, but many remain open until 8pm.

The initial results from Florida could favour Mr Biden due to the high volume of early ballots that the state began to scan more than three weeks ago; opinion polls suggest more Democrats voted early, whereas more Republicans waited until election day.

If there is a "blue mirage", it will fade as more in-person ballots from Tuesday are tallied.

7.30pm ET (8.30am Singapore time on Wednesday): Polls close in North Carolina, Ohio and West Virginia.

Like Florida, the initial results from North Carolina and Ohio could favour Mr Biden because the states began to scan early ballots weeks before election day. A truer picture of the vote will emerge as more ballots are tabulated.

North Carolina counts ballots that arrive as late as Nov 12 if they are postmarked by Nov 3. Ohio accepts ballots 10 days after the election if they are postmarked by Nov 2.

8pm ET (9am Singapore time on Wednesday): Voting ends in Alabama, Connecticut, Delaware, Florida, Illinois, Maine, Maryland, Massachusetts, Mississippi, Missouri, New Hampshire, New Jersey, Oklahoma, Pennsylvania, Rhode Island, Tennessee and Washington DC.

Pennsylvania does not begin to process early votes until election day and the state will accept mail-in ballots up to three days after the election if they are postmarked by Nov 3.

As a result, the initial vote counts from Pennsylvania may show a "red mirage" favouring Mr Trump until the absentee ballots are counted, experts say.

8.30pm ET (9.30am Singapore time on Wednesday): Reuters expects to publish updated national exit poll results from Edison Research, with percentage estimates of support for Mr Biden vs Mr Trump.

Polling stations close in Arkansas.

9pm ET (10am Singapore time on Wednesday): Voting ends in Arizona, Colorado, Kansas, Louisiana, Michigan, Minnesota, Nebraska, New Mexico, New York, North Dakota, South Dakota, Texas, Wisconsin and Wyoming.

Like Pennsylvania, the early results from Michigan and Wisconsin are expected to favour Mr Trump because ballots cannot be counted before election day. (Michigan does allow some ballots to be opened, but they cannot be counted.)

Arizona allows ballots to be scanned 14 days before the election.

10pm ET (11am Singapore time on Wednesday): Polls close in Iowa, Montana, Nevada and Utah.

Iowa allows ballot envelopes to be opened on the Saturday before the election and tabulating to begin on Monday.

Ballots postmarked by Nov 2 can arrive as late as the Monday after the election.

Nevada allows ballot scanning to begin 14 days before the election, and accepts ballots up to seven days after the election if they are postmarked by Nov 3.

11pm ET (12pm Singapore time on Wednesday): Voting ends in California, Idaho, Oregon and Washington.

12am ET (1pm Singapore time on Wednesday): Polls close in Hawaii.

NOV 4

1am ET (2pm Singapore time): Voting ends in Alaska.

DEC 8

States have until this date, known as the "safe harbour" deadline under federal law, to resolve any disputes over their vote totals and certify the winner.

If a state fails to finalise its vote count by then, Congress is no longer required to accept its results under the Electoral College system.

DEC 14

Members of the Electoral College cast their ballots for president. The candidate who receives a majority of the 538 electoral votes available, or 270, wins the presidency.

In all but two states, the winner of the state's popular vote earns all its electoral votes, which are apportioned by population.

In Maine and Nebraska, the statewide popular vote winner is awarded two electoral votes, and the remaining electoral votes are allocated to the popular vote winner in each of the state's congressional districts.

JAN 6, 2021

Congress meets at 1pm ET in Washington to count the electoral votes and declare a winner.

JAN 20: Inauguration Day

The winner and his running mate are sworn in as president and vice-president at the US Capitol in Washington.

For live updates and results, follow our US election live coverage.