Over 540,000 mailed-in ballots in 23 states were rejected during primaries this year, including in several battleground states, The Washington Post reports. With 20 states expanding mail-in voting in response to the ongoing coronavirus pandemic, the rate at which mailed ballots are rejected has the potential to become a major factor in deciding the outcome of November’s presidential election— particularly if it is a close race. In 2016, President Donald Trump won Michigan, Pennsylvania, and Wisconsin by about 80,000 votes. The same states discarded about 60,480 ballots during the 2020 primary season. “If the election is close, it doesn’t matter how well it was run—it will be a mess,” Charles Stewart III, an MIT political science professor, told the Post. “The two campaigns will be arguing over nonconforming ballots, which is going to run up against voters’ beliefs in fair play.”