President Donald Trump may have a lead in the popular vote and Electoral College as the early returns come in on election night, but his advantage could slowly disappear as tens of millions of mail-in ballots are counted. This could give Democratic rival Joe Biden the upper hand in the race for the White House, according to a new national survey led by researchers from Northeastern, Harvard, Northwestern, and Rutgers.

Mail ballots will likely play a large role in determining the winner of the battleground states of North Carolina, Ohio, Pennsylvania, Wisconsin, and Michigan, either because those states do not begin counting votes until the day of the election, or because they accept late-arriving ballots.

“The unprecedented logistical challenge of counting for all states, with by our estimate nearly 79 million votes arriving by mail—for most states, more than 3.5 times more than 2016—combined with the fact that some states do not start counting mail-in ballots until Election Day, and the certainty of millions of late arriving ballots, means that there will likely be a sizable and systematic shift in the vote count toward Biden after Election Day,” researchers write in their report.

In Ohio, for example, where polls show Biden ahead slightly, absentee ballots may be scanned prior to the election, but the count can’t be disclosed before polls close at 7:30 p.m. ET. In Florida, where Biden also has a lead, mail ballot counting can begin at 7 a.m. ET 22 days out from Election Day, but releasing the results early is a felony.

In some cases, the shift in Biden’s favor is expected to be dramatic, because more Democrats than Republicans are expected to vote via absentee ballot, the report finds.

“As of now, we estimate that Biden’s overall support is 20 points higher among those who are very likely to vote by mail than among likely voters who do not say they are very likely to vote by mail,” the report says.

The online survey of 20,000 U.S. residents was conducted September 4-27, and is the tenth in a series of surveys since April examining attitudes and behaviors regarding COVID-19 in the United States. 

It is not uncommon for the paced, orderly counting of mailed-in votes to change the outcome of a race. In the 2018 midterm elections, for example, Democrats’ net seat gain in the House of Representatives increased from 26 to 41 after Election Day because of the slow counting of votes that had arrived by mail and late-arriving ballots, the report notes.

The survey results could be good news for Biden, who leads Trump nationally among several polls. But the findings don’t come without risk for the former vice president, says David Lazer, University Distinguished Professor of political science and computer and information sciences at Northeastern, and one of the researchers who conducted the study.

“There could be problems around disqualifying ballots,” he says, comparing it to a situation in Massachusetts where election officials rejected nearly 18,000 mailed primary ballots this year for such reasons as arriving late or a missing signature.

In the case of Biden’s birthplace of Pennsylvania, ballots must be placed in a “privacy envelope” which is then placed inside the mailing envelope. Failure to place a ballot in the privacy envelope will result in the ballot being discarded.

For Trump, whose supporters overwhelmingly prefer in-person voting, according to the survey, the risk may come in the form of bad weather around the country: rain or even a blizzard, in a swing state like Wisconsin, that could make it hard for his base to get to the polls.

The survey doesn’t forecast a Biden victory. Its aim is to look at the states where dramatic vote shifts could occur in his favor based on states’ different rules for counting mailed ballots.

It is possible, given the dynamic state of the race, that the president’s cushion on election night in several states will be enough for him to hold a lead. Or Biden might win by such a large margin that it is clear that he is victorious on election night.

However, Lazer notes: “In any scenario, we anticipate a substantial shift toward Biden in a number of the competitive states after Election Day.”

To prepare for Nov. 3 and the days after, Lazer and his fellow researchers encourage the news media to set expectations that shifts will occur, sometimes abruptly. And they called on political leaders to be mindful of the impact their comments may have on the election outcome’s legitimacy.

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