New data reveals that absentee ballots are taking longer to reach election offices in key swing states than in the rest of the country. Over the past five days, the rate for ballot delivery in 17 postal districts that represent 10 battleground states was 89.1% – 5.9 percentage points lower than the national average. In other words, more than 1 in 10 ballots are arriving outside the United States Postal Service’s guaranteed one-to-three-day delivery window for first-class mail.

Specifically, in Detroit, Michigan, which includes some of the state’s largest concentrations of black voters, only 72.8% of ballots were delivered on time over the past five days. And in Pennsylvania, the on-time rate was a shocking 55.82% in Philadelphia.

If tens of thousands of ballots fail to arrive on time, they will be invalidated in the 28 states that require ballots to arrive by Election Day, regardless of when they were postmarked. And in such crucial battleground states, the absence of a significant number of ballots could affect which of the two candidates wins those electoral votes.

In response to these startling numbers, the Postal Service stated that “the data does not produce accurate, reliable information, as it is incomplete, subject to change, and overall, is not an accurate representation of the Postal Service’s performance.”