As Pennsylvania voters begin casting perhaps two million-plus mail ballots, Democrats and Republicans are in furious legal combat over a once-overlooked aspect of voting remotely: which ballots are counted, which are rejected as defective and which ones voters are allowed to correct.

Simple math explains why. In the 2020 presidential contest, Pennsylvania election officials rejected more than 34,000 mail ballots. In a tight 2024 election in the most coveted swing state, even a fraction of that many rejections could spell the difference between victory and defeat — not just in the presidential race, but also in any number of others.

What’s true in Pennsylvania is true, to varying degrees, in other battleground states. Michigan rejected more than 20,000 mail ballots in 2020 and even more in 2022; Arizona turned down 7,700; Nevada 5,600; and Wisconsin about 3,000.

But those states have relatively hard and fast rules governing the counting of mail ballots. Pennsylvania is an outlier: It lets partisan election boards in 67 counties interpret many already murky laws on accepting mail ballots, and even lets them decide whether voters should be allowed to fix mistakes.

Pennsylvanians cast more than 6.9 million ballots in the 2020 presidential contest. Experts predict that about a third of ballots in this year’s general election — 2.3 million or so — will be cast by mail.

“It’s quite possible that the election will come down to Pennsylvania, and, if it does, it could be a couple thousand votes,” said Charles Stewart III, who leads the Election Data and Science Lab at the Massachusetts Institute of Technology. “It could come down to a recount where the two parties are arguing one ballot at a time.”