Casting a ballot by mail isn't a new way to vote, but it is getting fresh attention as the coronavirus pandemic upends daily life.
The voting method is quickly becoming the norm and quickly becoming politically charged as some Republicans — specifically President Trump — fight against the mail-voting expansion happening nationwide.
Here are answers to key questions about mail ballots and the controversy around them.
The phrases "mail voting" and "voting by mail" encompass a wide range of policies, all meant to offer more flexibility to voters who either want or need to cast a ballot in a location other than a polling place.
The practice dates back to the Civil War, according to the Massachusetts Institute of Technology's Election Data Lab, when soldiers were given the opportunity to vote from the battlefield. States began expanding absentee voting laws in the late 1800s to accommodate voters who were away from home or sick on Election Day.
Currently, all states allow at least a portion of their voting population to vote by mail.
Some states allow all registered voters to receive a mail ballot (also known as an absentee ballot), and some states require an excuse or reason. Policies vary widely state by state as well as on what is required to have a mail ballot counted; some states only require a voter's signature, whereas others require witness signatures or the notarization of a ballot return envelope.
Almost all of them. Forty-six states now offer access to some form of mail voting to all voters, according to a recent report from the nonprofit Open Source Election Technology Institute.
Overall, that expansion has been bipartisan; 24 of the states have Democratic governors, and 22 have Republican governors.
But the limited pushback to the mail-voting expansion has come almost exclusively from Republicans.
All four states that have not expanded mail-voting access are led by Republican governors, according to the Open Source Election Technology Institute report.
Texas, for instance, is one of the four states that hasn't expanded access to mail ballots in response to the pandemic, and Republicans there are engaged in high-profile court battles to keep it that way.
Texas Attorney General Ken Paxton has said in statements that "fear of contracting COVID-19 does not amount to a sickness or physical condition as required by state law." He also said that his office would prosecute people for voter fraud if they use a mail-in ballot in a matter he said is improper.
Most other state-level Republicans have embraced mail-voting expansions however, including in a number of states such as Georgia and Iowa, which mailed all registered voters absentee ballot request forms ahead of their June primaries.
Overall, experts estimate as many as 70% of all ballots cast in November's general election could be cast by mail.
Overall, even before the pandemic sent election officials scrambling to make sure people could vote safely, mail voting has been on the rise in the United States.
About a quarter of all voters voted by mail in the 2018 midterms, more than double the rate of mail voting from 20 years ago. Much of that increase comes from the handful of states transitioning to all-mail elections in recent years.
Nationwide, the attitude toward mail ballots is also decisively positive. A recent Pew Research Center poll found that more than 70% of Americans think any voter who wants to vote by mail should be able to do so.
That includes 49% of Republicans. GOP support jumps to almost 70% in states where a sizable amount of the population already votes by mail. This seems to indicate that as voters become familiar with how mail voting works, they become more likely to support it.
The opposition to mail voting is anchored by Republicans in states that don't offer wide access to mail voting already.
The president has made it clear that despite voting by mail himself multiple times, he does not support wide expansion of the practice.
He has tweeted a number of false claims about what states are doing, including saying it was illegal for the secretary of state of Michigan to mail all registered voters in that state ballot request forms, and misrepresenting California's decision to mail all registered voters ballots.
Notably, Twitter decided to weigh in by adding a "fact check" label to his tweet about California's decision.
Trump says voters should be required to have a reason for voting absentee, otherwise the election process will become "substantially fraudulent." He and his staff point to a number of isolated incidents of fraud but have never provided the data needed to prove the point. Election experts say the claim is false.
Trump has also indicated he thinks expanding mail balloting would be bad politically for the Republican Party, although research disputes this notion as well. A recent study from Stanford University found no partisan effect of implementing universal mail voting.
Many Republicans in Congress, meanwhile, are focused on how mail ballots are returned. They don't want people to be able to return ballots for others.
Generally, Democrats see the practice as helpful for vulnerable populations; one person turning in all the ballots for a nursing home or housing complex for instance. But what Democrats call "community ballot collection," Republicans deride as "ballot harvesting," and say it opens up those same vulnerable populations to manipulation and vote coercion.
"These harvesters picking up ballots don't have to show an ID, they don't have to be a citizen, and they don't have to be eligible to vote," U.S. Rep. Rodney Davis, R-Ill., ranking member of the Committee on House Administration, wrote in a recent op-ed. "You expect Americans to believe that having someone who can't vote picking up ballots won't invite fraud in our elections?"
Democrats, however, are suing to allow to allow third-party collection in a number of states, including battlegrounds such as Pennsylvania and North Carolina.
While election experts say fraud in mail balloting is slightly more common than in in-person voting, it's still such a minuscule amount it's not statistically meaningful.
Amber McReynolds, a former Colorado election official and now the CEO of the National Vote at Home Institute, and Charles Stewart, director of the MIT Election Data and Science Lab, recently put the numbers in context in an op-ed in The Hill titled "Let's put the vote-by-mail 'fraud' myth to rest."
Over the past 20 years, they write, more than 250 million ballots have been cast by mail nationwide, while there have been just 143 criminal convictions for election fraud related to mail ballots. That averages out to about one case per state every six or seven years, or a fraud rate of 0.00006%.
"Expanding voting by mail will be a challenge in most states in 2020," they write. "But we reiterate: There is no evidence that mail-balloting results in rampant voter fraud, nor that election officials lack the knowledge about how to protect against abuses."
There's also no evidence of the sort of mail-voting interference that U.S. Attorney General William Barr mentioned in a recent New York Times Magazine interview.
Barr alluded to foreign countries being able to "easily make counterfeit ballots, put names on them, send them in," although election officials and experts immediately debunked that assertion.
Jennifer Morrell, an elections consultant and former local elections official in Utah and Colorado, told NPR that for such a plot to work just for a single ballot, an adversary would need to mimic everything perfectly from the ballot's size, style and even paper weight to the envelope it's mailed in — all of which often changes every election cycle and which is different from county to county.
"Ballots are built unique for each election. Each jurisdiction will normally have dozens to hundreds of unique ballot styles. Proofs for each ballot style are reviewed and tested to ensure the ballot scanners will read those ballots and only those ballots," Morrell said. "Even ballots created on that system from a previous election cannot be read."
In all, Morrell listed dozens of unique aspects that the adversary would have to copy perfectly in addition to lining it up with an actual voter in a registration system and faking a signature that aligns with the signature on file for the voter.
"You would need to replicate all of these elements exactly and do it for the 10,000-plus jurisdictions, and hundreds of thousands of unique ballot styles within the U.S.," she said.
Or as Colorado elections director Judd Choate put it to The Washington Post: "It's preposterous to the point of humor."
This idea is popular with some Democrats in Congress, including Sens. Ron Wyden and Amy Klobuchar, who introduced the Natural Disaster and Emergency Ballot Act earlier this year. The bill would drastically expand mail-voting access to all U.S. voters and reimburse local election officials for the multibillion-dollar cost of providing that access.
But it's unlikely to pass as Republicans in Congress are resistant to any measures that regulate elections from the federal level; they insist that states and localities should govern their own democratic processes.
Election experts are also skeptical that states, especially ones used to only mailing and receiving a small number of mail ballots, could effectively scale up their operations in time for November. Such changes usually take states years, if not decades.
The private companies tasked with printing the ballots could also be overwhelmed at the rapid increase in demand.
"Decision-making needs to be right now," said Kevin Runbeck, CEO of Runbeck Election Services, one of the nation's largest ballot printing companies, at a recent U.S. Election Assistance Commission hearing. "We cannot gear up, we cannot build equipment fast enough if you wait until July to place your orders."