States in play for both candidates in this year’s presidential election all allow voters to cast their ballots by mail, a process that has attracted controversies and legal battles in the final weeks of campaigning. That includes Pennsylvania, Michigan, and North Carolina — each of which has seen concerns raised over absentee ballots.

These swing states see varying degrees of mail-in voting, and campaigns, political operatives, and election policy hawks are closely tracking the vote-by-mail process because of how it could affect the election outcome.