Over the past five days, the on-time rate for ballots in 17 postal districts representing 10 battleground states and 151 electoral votes was 89.1 percent — 5.9 percentage points lower than the national average. By that measure, more than 1 in 10 ballots are arriving outside the Postal Service’s one- to three-day delivery window for first-class mail.

Those delays loom large over the election: 28 states will not accept ballots that arrive after Election Day, even if they are postmarked before. Continued snags in the mail system could invalidate tens of thousands of ballots across the country, and could factor into whether President Trump or Democratic nominee Joe Biden captures crucial battleground states and, ultimately, the White House.

In Michigan, for example, the Detroit postal district — which includes some of the state’s largest concentrations of Black voters, who are crucial to Biden’s campaign — delivered only 72.8 percent of ballots on-time over the past five days, according to Postal Service data filed in the District Court of the District of Columbia. In the Greater Michigan district, which represents the rest of the state, 84.3 percent of ballots arrived to election officials on time.