In late April Democrat Kweisi Mfume of Maryland won a special election for a seat in the U.S. House of Representatives in a landslide victory against his Republican rival. More remarkable than his win, which garnered three quarters of the vote, however, was that more than 110,000 Baltimoreans in the district cast their ballot by mail. Only 1,000 voted at one of three polling centers open there. In Georgia’s primaries in early June, more than one million voters used mail-in ballots, a huge increase over previous elections. In Wisconsin, voters are suing for greater access to such ballots because many polling places have closed in response to COVID-19—which has made it much harder to vote in person in the state’s primaries.

These are just some of the political battles that have played out during the pandemic, when many people feel it is too risky to gather in public to do things such as vote in person. On June 2 eight states and Washington, D.C., held their primary elections, four of which had been postponed because of the disease. Half of the eight states had turnouts surpassing their 2016 levels—after taking extra efforts to ensure citizens received vote-by-mail ballots. But the others fell short despite such measures.

Public health experts predict the coronavirus will still be a threat this fall. Americans can expect that some social distancing recommendations, such as staying six feet away from others and avoiding large gatherings, will remain during the November presidential election, in which Donald Trump will face off against Joe Biden. Political scientists say the ability to vote by mail, as well as attitudes about the danger of voting in person—which vary by party affiliation—could swing the contest.

Election officials are implementing several fixes to allow people to vote safely, such as ramping up online registration and expanding mail-in voting. Rollouts of these measures across the U.S. have not been equal, however. And voting processes could differ from state to state come November. Some key swing states, such as Michigan, Pennsylvania and Wisconsin, do not automatically send out vote-by-mail ballots but require people to apply for them. Another 16 states only allow mail-in voting for people with a particular reason, such as a disability.

Republicans have long asserted that voting by mail favors Democratic contenders. (Trump has also claimed, falsely, that such voting encourages fraud. Evidence shows it does not.) But Democrats have countered that the opposition comes because mail-in ballots make it easier for minorities, immigrants and young people to vote, and those groups tend not to vote for Republicans. For example, North Carolina’s Republican-controlled legislature placed limits on early voting after it found that African-Americans were more likely to vote early and to vote for Democrats.

Theoretically, states with a robust vote-by-mail system and enthusiasm for ousting Trump could expect to see a blue wave in November, says political scientist Rachel Bitecofer of Christopher Newport University and the Niskanen Center, a think tank in Washington, D.C. Bitecofer’s predictions for the 2018 midterm elections were largely correct. Other political scientists say that if states do not implement expansive systems for such voting, Democrats—who, according to recent polls, are more fearful of the virus—might stay home. And that scenario would favor Republicans on the ballot.

The Campaign Trail

The pandemic will likely dampen large political rallies and conventions this summer, putting candidates in a tough spot: they have to keep Americans safe, but they also need to rally voters and build enthusiasm for their campaign. Trump insists that his rallies and the Republican National Convention will proceed despite the pandemic—and has now moved part of the convention to Florida after opposition to a fully attended event in North Carolina, reportedly without social-distancing measures, came from the latter state’s governor. Biden, however, has largely moved his campaign online and plans for the Democratic National Convention to remain fluid.

“I expect both campaigns will structure their campaign events to fit into their overall narrative about how the county should be responding to the COVID-19 crisis,” says Marc Meredith, a political scientist at the University of Pennsylvania. “I also expect that the conflict between Trump and North Carolina’s governor over the [Republican convention] is a preview of the conflict that will repeatedly play out between the Trump campaign and state and local governmental officials—especially [in] Democratic states.”

But Meredith says that campaigning’s impact on voter turnout is often overstated. A long-standing political science doctrine, called the minimal effects hypothesis, holds that presidential campaigns have a marginal effect on voting decisions. He does worry, however, that the pandemic will limit voter registration. Typically, volunteers with registration forms mill around political rallies, set up tables at college campuses or knock on doors in neighborhoods. Such in-person tactics will be very much reduced. “Door-to-door canvassing is one the most effective ways to mobilize nonvoters to become voters, as it generates social pressure,” Meredith says.

A 2000 Yale University study found that personal canvassing substantially increased voter turnout. According to Bitecofer, such voter-registration efforts are key in deciding elections. Her model holds that turnout, not swing voters, ultimately determines who wins and who loses. With this situation in mind, campaigns will likely move to mobilize alternative techniques to target new voters by phone and online. Whichever camp can do so better could shift election results in its favor.

Vote at Home

Voting for president, unlike campaigning, cannot be done online. Mail-in ballots seem to be the obvious cure for an election plagued by a pandemic. Though studies of past elections indicate that voting by mail did not affect turnout to favor either Democrats or Republicans, 2020 is tilting that notion on its head.

Thad Kousser, a political scientist at the University of California, San Diego, says he used to be dubious of the claim that mail-in voting hurts Republicans. But after he conducted a recent survey on electoral processes and the coronavirus—results were released in a preliminary, non-peer-reviewed paper on April 30—Kousser amended his stance. “I think we are seeing evidence of an emerging trend in which a new partisan divide is opening up over voting by mail,” he says.

When Kousser asked both Republicans and Democrats about their most preferred method of voting, in absence of any information on the coronavirus, a higher percentage of Democrats said they wanted to vote by mail. When he asked the same question accompanied by scientific projections about the spread of the coronavirus, the partisan gap doubled. “Democrats and independent voters were much more likely to say they wanted to vote by mail after being reminded of COVID-19,” Kousser says. “But the Republicans don’t seem to be influenced by the scientific projections. There is clearly polarization when it comes to whether people believe COVID-19 is a significant risk and want to change their behavior.” He stresses, however, that his survey looked at people’s most preferred voting method and did not suggest that Republicans absolutely would not vote by mail. Rather it showed that if given an array of options, more Democrats than Republicans would choose mail-in voting during the pandemic.

Five states—Oregon, Colorado, Utah, Washington and Hawaii—have already established all-mail voting, in which every resident is mailed a ballot automatically. In another 29 states and Washington, D.C., residents must actively request a mail-in ballot from election officials but do not have to give a reason. The six swing states of Florida, North Carolina, Arizona, Michigan, Pennsylvania and Wisconsin all belong to this second group. The rest of the states have rules that only allow such voting if a citizen has a specific excuse, such as age, illness or disability. But several of them say they will allow more mail-in voting this year because of the pandemic.

These policies, particularly in the swing states, means a vote-by-mail partisan divide could significantly influence who becomes president. If less supportive Republicans don’t request a ballot in time or are unable to go to the polls in sufficient numbers, the Democratic candidate could claim victory in November. To underline this point: In one hard-fought primary race for Wisconsin’s Supreme Court, mail-in ballots favored a liberal candidate over a conservative one and suggested liberal Democrats voted by mail while conservative Republicans voted in person. A New York Times analysis found that the liberal-leaning candidate challenging for the seat generally performed at least 10 percentage points better than the conservative incumbent in such ballots. At one precinct in particular, the conservative justice garnered 64 percent of the Election Day vote, while the challenger took 70 percent of the votes cast by mail. Those mail-in ballots helped to carry the race for the liberal.

Voting by mail may be especially important in November if the number of physical polling stations are reduced because of COVID-19. In the Wisconsin primary, only five of Milwaukee's typical 180 polling sites were open. Getting enough poll workers to sites may add to the trouble. More than half of poll workers in the last two general elections were age 61 or older, according to the Pew Research Center. And in the 2018 election 27 percent of such workers were older than 70. This group faces the greatest threat from the coronavirus and has been told all year to stay home to be safer. In both Wisconsin’s and Georgia’s recent primaries, fewer than usual workers showed up. The concern is reasonable: since the primary in Wisconsin, state health officials have identified 71 people who contracted the coronavirus from polling centers.

Mail Trouble

To make mail-in voting work well for the November election, infrastructure will need to be ramped up in the 45 states that have not been automatically mailing ballots, says Amber McReynolds, CEO of the National Vote at Home Institute, a nonpartisan research group that advises election officials. She adds that such efforts to bolster vote by mail need to start “immediately.”

“There are a lot of parallels with the COVID-19 virus in terms of medical equipment,” McReynolds says. “If we’re not prepared, and in many ways the health care system wasn’t adequately prepared, then we’re scrambling to do it at the last minute. States need to evaluate what their needs are now and put those orders out.”

States need to order things such as counting equipment (high-speed scanners that can process and count a huge influx of absentee ballots), new printers to produce mail-in ballots en masse and possibly ballot-sorting technology with signature verification, she says. Officials also need to ensure that Native-American voters on tribal lands, who may lack access to reliable mail service, can vote at sanitized, mobile polling units.

Without the right equipment and procedures, there is already evidence from the primaries that states will run into trouble in November. In Indiana’s June 2 primary, state officials allowed residents to vote by mail. But some people who requested absentee ballots never received them. Election officials in Washington, D.C., also struggled to fulfill all the requests they received for mail-in ballots, forcing voters to wait in long lines at just 20 in-person polling stations that remained open. And in Pennsylvania, which passed legislation last year to move to a no-reason-needed mail-in approach, the system buckled under increased absentee-voting pressure. Thousands received the wrong ballot. And more than a week later, votes remained uncounted.

Senate Democrats requested $2 billion in March for infrastructure to expand online voter registration, early voting and voting by mail. The amount was opposed by Senate Majority Leader Mitch McConnell of Kentucky. And in the end, the Senate passed a bill offering just $400 million for such expansion.

A malfunctioning vote-by-mail system during the general election, when turnout will be much greater, could hurt Biden by making in-person voting more important. A preprint paper released in March found that Republicans were less likely to practice social distancing measures than Democrats, perhaps because of the same skepticism about projections of disease spread that Kousser found. “If Democrats are taking more precautions because of the coronavirus, we may see them not turn out as much as they would have without the pandemic,” says Shana Kushner Gadarian, the study’s co-author and a political scientist at Syracuse University. “That is pretty disturbing.”

What the virus will actually do to November turnout for each candidate will not be known until after Election Day, of course. The early evidence from primary elections, combined with the results of national surveys, indicates that an expansion of voting by mail will favor Biden. Strong voter registration efforts by the Republican Party could still offset that advantage. And if states are unwilling or unable to bolster mail-in voting infrastructure—particularly in the six battleground states—and enough Biden supporters are too worried about the disease to vote in person while his opponent’s voters feel unhindered, the pandemic could pave the way for Trump’s reelection.

Read more about the coronavirus outbreak from Scientific American here. And read coverage from our international network of magazines here.