If the November 3 election between President Donald Trump and Joe Biden is close, many expect legal battles that, like in 2000, could go to the Supreme Court
US elections were easier before, when most people voted in polling stations and their choices were automatically tallied by machine.
This year, because of the Covid-19 pandemic, voting by mail is soaring, posing manpower, technical and legal challenges across thousands of election jurisdictions, each with its own procedures and rules.
If the November 3 election between President Donald Trump and Joe Biden is close, many expect legal battles that, like in 2000, could go to the Supreme Court.
Polls show that far more Democrats than Republicans are likely to vote by mail, and Trump's Republican party has launched scores of legal fights that aim to limit postal voting.
In recent elections around 1% of mailed ballots were rejected, and that figure is expected to rise with the surge of mail-in votes.
It could mean hundreds of thousands of disputed votes. The 2000 election was decided by a mere 537 vote difference in Florida.
What's the overall vote picture?
In 2016 about 139 million Americans voted, 33 million of them by mail. This year researchers project turnout could top 150 million, with possibly half of it mail-in votes.
How does mail-in voting work?
Nine states and Washington, DC automatically send mail-in ballots to all voters. In other states, voters have had to request them.
In the past this was restricted to "absentee" voters, but this year many states -- though not all -- made it possible for anyone to get an absentee or mail-in ballot.
Each state has its own rules. Most require the voter to fill out the ballot, put it in a return envelope, sign the outer envelope and mail the vote back, or drop it in designated drop boxes.
But some states include a privacy sleeve, which the ballot first goes into, before placing it in the envelope.
Some states require a voter to have witnesses also sign the outer envelope and provide their contacts. In Alabama, which has some of the most restrictive voting laws, the voter needs two witness signatures.
When are ballots counted?
In-person votes are tabulated automatically, and in most cases are ready to announce within hours or even minutes of polls closing. But mailed ballots involve a laborious process and each state again has its own rules.
Some states will only count mailed ballots that arrive by election day; others will accept them up to 10 days later if they are postmarked by election day.
Because of the burden on the Postal Service, some states have lengthened the period they will accept ballots.
The process for verifying signatures, opening envelopes, and removing and then counting the ballots differs from state to state.
In Colorado, for example, ballots are opened on receipt. Counting -- handled by machine -- begins 15 days before the election, but data cannot be revealed until 7:00 pm on election day.
One bottleneck is the postal service, which has seen cuts that some see as Republican attempts to hamper the mail-in vote.
Given the volume of mailed votes, it takes days to count them all.
Another bottleneck is signature verification. In some states, this is an automated process, but in others it is done manually, by poll workers visually matching signatures with what the state has on file.
Many people's signatures change over time, and some have more than one way of signing. Younger people who grew up digitally, especially first-time voters, might not even have a regular signature, or not have one on file.
After court battles lasting for months, a North Carolina appeals court ruled in October that voters will be given a chance to fix their ballots.
In Pennsylvania following a Republican lawsuit, a court has ruled that "naked ballots," which could be in the tens of thousands, cannot be counted. But other states accept them.