As of Oct. 30, more than seven million mailed ballots had not been returned in the 13 most competitive states that require ballots to arrive on or before Election Day, a Wall Street Journal analysis of data from the University of Florida’s U.S. Elections Project found. That’s about 28% of the more than 24 million ballots that had been tallied in those states, the Elections Project data show.

Many of the states, including Michigan, Wisconsin, Arizona and Florida, were narrowly won in the 2016 presidential election, in some cases by only a few thousand votes, making the outstanding ballots potentially critical to figuring a winner this time around.

Mail delays won’t help the situation: So far this week, first-class mail times have slowed from the week before in 12 of the 13 competitive states with tight deadlines, according to data from mail-tracking firm GrayHair Software…

The total number of outstanding votes is difficult to know with precision. Some voters who requested mail-in ballots may wind up voting in person or not at all. Some states, such as Colorado, automatically mail ballots to every eligible voter, leading to potential overestimates in the number of outstanding ballots. Many states, including Florida and Arizona, also offer ballot drop boxes, which don’t use the Postal Service.