But amid the swelling turnout is growing concern over the yawning gap between absentee ballots that have been requested and those that have been returned. With just days to go, 36 million ballots that were requested have either not been returned or have been rejected. Many of those ballots could still be in the mail or in processing or might have been sent to people who now plan to vote in person.

Any problems with the early vote are also likely to affect Democrats more than Republicans. In almost every state, Democrats have requested absentee ballots at a higher rate than Republicans. In Pennsylvania, nearly two million registered Democrats requested absentee ballots, compared with fewer than 790,000 Republicans. And while 70 percent of those Democratic voters have returned their ballots, roughly 590,000 ballots sent to registered Democratic voters have not yet been returned, along with 360,000 ballots sent to registered Republicans.

Voters in Pennsylvania, one of the most important battleground states, have been increasingly unnerved by the flurry of litigation regarding the deadline for when ballots can be accepted. The Supreme Court left open a possibility of a future ruling on ballots that are postmarked by Election Day but arrive late, and the secretary of state told all county election officials to segregate those ballots.

Worries about the U.S. Postal Service have added to the anxiety. The agency said in a filing that staffing issues resulting from the pandemic were causing problems in some facilities, including some in central Pennsylvania. Only 78 percent of employees are available, according to the filing.