Something has been happening in Pennsylvania elections.

When a Democrat leads on election night, that advantage grows as absentee and provisional ballots are counted in the days after. When a Republican leads, that edge shrinks.

It happened in 2018, when Gov. Tom Wolf and Sen. Bob Casey, both Democrats, saw their winning margins grow by more than 28,000 votes after election night. In each of the last four presidential elections, the tallies moved in favor of Democrats by more than 22,000 votes after Election Day.

It’s called the “blue shift.” It’s a little-known phenomenon researchers have identified only in the last few years. And it could further complicate the already complex task of declaring a winner on election night, in a critical swing state that helped elevate Donald Trump to the presidency in 2016 and that is again expected to be close in 2020.

In short, as more voters began using nontraditional voting methods such as absentee and provisional ballots in recent years, a partisan split emerged in many states: Democrats tended to vote using those methods more often than Republicans.

That means the blue shift will only become more significant, voting experts say, as more and more ballots are cast outside of polling places. And it may take on particular importance in Pennsylvania, where a new election law means anyone can vote by mail starting in this year’s presidential primary — a dramatic expansion from the previous system, which required a justification for why voters couldn’t appear in person. The number of absentee ballots is expected to balloon this year and continue growing after that.

If the blue shift continues, that would mean election night leads — which are tabulated using the initial returns from precincts and often don’t include other votes — could be narrowed or even flipped in high-profile races, including for president.

“This is the new normal,” Edward B. Foley, the Ohio State University law professor who first identified the blue shift in a 2013 paper, said in an interview last week. “This is how we conduct elections now, in a world with absentee voting on demand. … We just have to be patient, let the system work, let the ballots be counted, and whoever wins the authentic votes is the winner.”

None of this changes the official, certified results that already come in long after the world knows who won. But being able to call a race in the hours after polls close has become an expectation for the public, with news organizations building sophisticated models for projecting winners based on unofficial results as they come in.

As unofficial election night tallies become even less complete than they are now, voting experts fear the spread of misguided claims of fraud and malfeasance: Will voters and candidates, accustomed to knowing the winner on election night, accept the ever-changing numbers as they roll in over several days? Or will they quickly jump to accusations of foul play?

“That’s a real worry,” Burden said. “Once partisan politicians start to make accusations on election night, there’s really not much going back in terms of bringing the public back in terms of a reasonable discussion of what’s happening.”

As Foley put it: “No one’s trying to steal an election or cheat or anything like that. It’s now an additional feature of our system, but it’s one we haven’t completely psychologically caught up with.”

Why the ‘blue shift’ occurs

Electoral systems have changed significantly over the last two decades, with states adopting a series of reforms meant to make voting more accessible. The spread of provisional ballots, under federal law, was meant as a fail-safe for voters encountering trouble at the polls, and expanded access to absentee ballots was meant to make voting more flexible.

The reasons for the partisan split that emerged in who uses those methods are still being studied, election experts said. But factors appear to include that young voters, voters who move often, and low-income voters who use absentee and provisional ballots tend to lean more Democratic than other voters.

“There is nothing inherently blue or red about this, by law or by design,” Foley said.

In fact, he said, some states may experience a “red shift,” and even Pennsylvania’s blue shift may change in the future as different voters adapt to the new absentee ballot system and counties that used to process them on election night stop doing so.

Why the ‘blue shift’ has become more important

The number of mail-in votes, in particular, has grown significantly.

While absentee ballots used to be meant largely for voters who were literally absent on Election Day, states began to aggressively expand mail-in voting over the last two decades. Some states, including Oregon and Colorado, conduct elections almost entirely by mail.

Pennsylvania, like some other states in the Northeast and Mid-Atlantic, largely resisted that movement, which was pioneered in Western states. But Wolf reached an agreement last year with Republicans who control the state legislature to allow all voters to qualify for mail-in ballots.

Absentee ballots have long been about 5% of votes cast in Pennsylvania or even less. Election officials expect that number to grow to 15% or even 20% this year.

And many counties won’t count those absentee ballots on election night, meaning a significant number of votes will be left off the unofficial election night tallies.

Why the ‘blue shift’ worries experts

Voters and even candidates who don’t understand the changing nature of absentee and provisional ballots may think evaporating margins are evidence of malfeasance. Experts warned this becomes more likely in highly partisan environments when people are already primed to see wrongdoing.

For Democrats, this might mean jumping to conclusions about foreign interference. For Republicans, it may trigger worries about voter fraud.

In the 2018 Senate race in Arizona, Republican candidate Martha McSally held a narrow lead on election night, but Democrat Kyrsten Sinema ultimately won. Before the Associated Press called the race six days after the election, President Donald Trump took to Twitter to raise the specter of “electoral corruption.”

Foley and others said raising awareness of the blue shift can help inoculate people against unfounded claims.

“I’d hate for this misinformation about the process or this misunderstanding of the process [to contribute] to the anxiety,” Foley said. “If we think of this ahead of time and put it out there, then maybe we can avoid the problem. If you don’t plan for a hurricane, you may not be able to handle it properly.”

There’s one other thing people can do to prevent this kind of trouble, said David C. Kimball, a political science professor at University of Missouri-St. Louis.

“Get your states to provide more funds so counties can count them as fast as you want,” he said. “Election administration is vastly underfunded."

Barring that, Kimball said, people will just have to be patient.

“I just want the count to be accurate and correct, and if I need to wait a few days or a week after the election to get that, I’m willing to wait,” he said. “We’re just going to have to all get used to this, of not knowing the winner on election night.”