An electoral worker is seen during the vote-by-mail ballots scanning process in Miami, Florida USA

An electoral worker prepares to process ballots at the Miami-Dade County Election Department in Florida, a battleground state with a history of contested election results.Credit: Chandan Khanna/AFP via Getty

As election day approaches and COVID-19 cases surge in the United States, debate rages over how Americans should vote fairly and safely.

Because of the pandemic, a record number of votes are expected to be cast by mail before 3 November. A number of states, including New Jersey, are sending ballots to everyone for the first time (see ‘The postal experiment’) — and in a July poll, almost two-thirds of respondents said they were likely to use mail-in ballots this year. Just one in four voters cast a postal ballot in the past two federal elections.

But US President Donald Trump has regularly attacked mail-in voting, referring to it as a “whole big scam”. Polls this year have shown that Democrats favour postal ballots more than do Republicans, whose party Trump represents.

Until 2020, both the Republican and Democratic parties supported postal voting, says Michael Barber, a political scientist at Brigham Young University in Provo, Utah. “Now this issue has become really, really partisan.”

So, will the move to postal voting increase voter turnout? And could it tip the balance in favour of either political party? Will voting by post increase the risk of fraud, as Trump has suggested?

Nature looks to the research for answers.

Will adoption of postal voting affect 2020 voter turnout?

Typically, some 60% of people eligible to vote in US presidential elections do so. Those who don’t vote give reasons including a lack of paid time off on election day, and long queues at polling places.

But research suggests that mail-in voting could increase overall voter turnout.

Barber has found1 that in presidential and midterm general elections between 1992 and 2018, counties in some states that switched to mandatory postal voting saw an increase of between 1.8 and 2.9 percentage points in the number of residents who voted. And a separate study from Colorado showed that the state’s decision to implement all-postal voting in 2014 increased voter participation by 9.4 percentage points.

Infographic: The postal experiment. Map of the USA showing the mail-in ballot status for each state.

The researchers involved note that the increased turnout came from people in specific demographics. “There was a disproportionate increase for people who have historically faced greater barriers to voting: young people, Black voters, people with less education and less wealth,” says Charlotte Hill, a public-policy researcher at the University of California, Berkeley, and a co-author of the Colorado analysis.

What’s more, long queues — one of the barriers to voting cited by non-voters — can be more prevalent in disadvantaged communities, which generally lean toward Democrats. On the first day of early voting in Georgia in 2020, people in some areas of Atlanta where most residents are people of colour waited for as long as ten hours to cast their ballots, because of an unexpectedly large turnout and a few technical glitches.

Is postal voting really more susceptible to fraud than in-person voting?

Fraud is exceedingly rare in mail-in voting, thanks to a range of security measures — including identity verification, ballot tracking and the efforts of the US Postal Service police force.

Oregon, the first state to adopt voting exclusively by post, has sent more than 100 million ballots to registered voters since 2000. Only about a dozen cases of proven fraud have been documented in the state, according to the Brennan Center for Justice, a nonpartisan law and policy institute in New York City.

Hill points out that postal ballots use a wide variety of designs and materials, which makes it difficult to duplicate them effectively on a scale that would affect the outcome of an election.

The absence of fraud mirrors the overall state of affairs for voting in the United States. In 2014, Justin Levitt, a law scholar at the Loyola Law School in Los Angeles, California, tracked allegations of voter fraud across the entire country since 2000 and identified just 43 cases in more than one billion votes cast.

“You’re more likely to get struck by lightning than have someone impersonate you at the ballot box,” says Sonni Waknin, a fellow with the University of California, Los Angeles, Voting Rights Project and co-author of a report that debunks several concerns about voter fraud.

“What is more concerning is people being afraid of fraud,” she adds. This could prevent them from voting safely or from casting a ballot at all. “There’s a lot of misinformation about voting out there.”

Are postal votes less likely to be counted?

The short answer is yes, slightly — and undercounting could disproportionately affect people from certain groups, many of whom tend to vote Democrat.

“Voting by mail is a fundamentally different system of voting than voting in person,” says Daniel Smith, a political scientist at the University of Florida in Gainesville. “It is subject to the fluctuations, inconsistencies and delays of mail.”

In mid-August, the US Postal Service warned states that it could not guarantee that all ballots would arrive in time to be counted for the November election. But late arrival is just one of the reasons that election administrators might dismiss a postal ballot. Sometimes, ballots are discarded because of an error or inconsistency, such as the voter’s signature not matching the one on file. In 2016, approximately 1% of postal ballots received across the United States were rejected.

Research shows that ballots from certain demographic groups are rejected more often than from others — and that those groups are the ones that typically vote Democrat. In a study2 published in September, Smith and his colleagues analysed millions of postal ballots cast in the 2016 and 2018 general elections in Florida, and found that Hispanic and Black voters were about twice as likely as white voters to have their ballots rejected. Voters aged 18–21 faced rejection rates four times those of the average voter aged 45–64. Reasons include inexperience at the polls, research suggests. And younger voters might have changed their signatures since they registered to vote, or might wait until the last minute to vote.

Rejection rates and rejection disparities might increase in 2020, as more people vote by post for the first time. Smith looked at data from Florida’s primary election in March 2020, and found that people who had voted in person in the previous two major elections were almost three times as likely to have postal ballots rejected as were experienced postal voters — even after accounting for age, race, ethnicity, gender and political party.

“We are seeing a sudden influx of new voters who do not know how to use the system. We are, without question, going to have a higher rate of rejected vote-by-mail ballots,” says Smith. “Experience matters.”

Looking at the primary and previous two general elections in Florida, Smith found that independent of experience, Democrats had greater rejection rates than did Republicans. Democrats also submitted more postal votes than did Republicans, further widening the disparity in ballot rejections. Smith’s results have not yet been peer reviewed.

Some states are trying to reduce rejections: 18 now have laws that allow ‘curing’, or correction, of postal ballots. If officials find that a signature is missing or doesn’t match that on file, for example, they will notify the voter and give them an opportunity to fix the mistake — assuming the ballot arrives early enough.

Could the rise in postal voting affect who wins the election and when a winner is announced?

Data from past elections find that the increase in voter turnout resulting from mail-in voting has not given either Republicans or Democrats an edge3. But Barber suggests that the 2020 election could be different, especially given the Republican rhetoric against the practice.

So far, substantially more Democrats than Republicans have requested absentee ballots, and returned them, according to Michael McDonald, a political scientist at the University of Florida, who has compiled data from the 18 states that have made this information available.

But any advantage for Democrats could be offset by disparities in rejected ballots — and by a higher Republican turnout in person. In a poll of probable voters by The New York Times and Siena College Research Institute in Loudonville, New York, last month, 28% of Democrats said they planned to vote in person on election day, compared with 68% of Republicans.

Scholars agree that as a result, Trump will probably take an early lead on 3 November, because in-person votes are generally counted before postal ballots. Some states are beginning to count postal ballots early, and others are ‘pre-canvassing’ them — checking them for eligibility and preparing them for scanning — ahead of the election, so that they will be ready to count quickly. But other states will wait until election day to do any of that work — causing delays in delivery of the official result. “Be prepared to think of this as ‘election week’ or ‘weeks’ and not just election day,” says Hill.