The states that could play the biggest role in tipping the presidential election to Donald Trump or Joe Biden have also experienced some of the starkest delays in delivering ballots by mail, according to newly released data from the U.S. Postal Service.

The numbers show alarming slowdowns in several major cities and swing states, even if they paint a reassuring picture for the majority of the country.

At least one day this week, fewer than 85% of ballots arrived on time by USPS standards in postal districts in Atlanta, Detroit, greater Michigan, parts of Pennsylvania and North Carolina, and Lakeland, the district covering most of Wisconsin. For first-class mail, which is how ballots are categorized, on time means the mail has arrived within a delivery window of 1 to 3 days.

Coupled with efforts to invalidate ballots that arrive after Election Day, mail delays could disenfranchise an untold number of voters who received their ballots late, mailed their ballots too close to Election Day, or even followed the advice of voting advocates and mailed their ballots with a week to spare.

(Note: If you have not voted by this point, you should drop off your absentee ballot at an authorized location instead of relying on the mail, or you should plan to vote in person.)

Detroit, a crucial stronghold for Democrats that has suffered slow delivery times for years made worse by outbreaks of COVID-19 among its postal workforce posted some of the worst on-time scores for ballots in the country.

On Tuesday, only 51.65% of inbound ballots that is, ballots being mailed back to election officials arrived on time. By Thursday, that number had improved to 80.2% of ballots delivered on time, but that still made Detroit one of the worst-performing postal districts in the country.

By sheer numbers, Wayne County, which covers Detroit, is projected to provide Biden with a larger net share of absentee ballots than any other county in Michigan. Absentee ballots could increase his margins there by 15.4%, according to a recent analysis by the American Civil Liberties Union.

Delays around Detroit would also disenfranchise huge numbers of people of color. Nearly 56% of voters of color in Wayne County who are overwhelmingly Black say they plan to vote by mail.

These are counties where if absentee ballots are mistreated, if, say, there are excessive rejections, the risk of suppression of voters of color is high, said Lucia Tian, the ACLUs chief analytics officer.

Delays in Pennsylvania could also affect a huge share of people of color. Philadelphia and its three surrounding counties are expected to deliver Biden his largest share of absentee votes in Pennsylvania. In Philadelphia, as well as in Bucks, Delaware and Montgomery Counties, anywhere from 42% to 47% of people of color report plans to vote by mail. If all absentee ballots are counted, Bidens margins in the Philadelphia suburbs could be as much as 20% larger, according to a recent analysis by the American Civil Liberties Union.

Overall, mail delays are likely to be more damaging to Democrats than Republicans. Millions more voters are planning to cast ballots by mail than ever before due to the COVID-19 pandemic, with Biden supporters being far more likely than Trump supporters to say they plan to vote by mail. In the largest counties in Michigan, Pennsylvania, Georgia and Wisconsin, absentee ballots may swing Bidens margins by more than 20 points.

Republicans, knowing this, are fighting furiously for the federal courts to invalidate as many absentee ballots as possible, including by cutting off ballot deadlines at the last minute. For states that accept ballots that were postmarked by Election Day but arrive later, the Supreme Courts conservatives have signaled they may force states to throw those ballots out.

As of Friday afternoon, there were still at least 35 million absentee ballots that have yet to be returned to election offices, although some may belong to voters who are voting in person instead.

There are, however, limits to what all these numbers can tell. The data USPS released does not cover all ballots, including ballots the post office is delivering directly to elections officials. A ballot being late by postal standards does not mean it is too late by election standards.

And nationwide, the numbers support postal officials claims that they are taking extra steps to deliver ballots faster than the rest of the mail.

In addition to special measures like multiple daily sweeps at processing plants, the USPS Office of Inspector General is monitoring ballot processing in real time, according to Tammy Patrick, a senior adviser on elections to the nonprofit Democracy Fund who is in regular contact with the agency.

On Thursday, 95.2% of ballots that the USPS was tracking arrived on time; a day earlier, the total on-time score was over 97%. And places like Atlanta, Lakeland and the Carolinas, which had on-time rates as low as 41%, 56%, and 76% at points this week, all saw on-time delivery rates improve to 95% by Thursday, the last day data was submitted.

This is the first set of data the USPS has released to the public to show how quickly ballots are moving in specific regions. Last week, the agency released aggregate figures for the country that showed that ballots have a higher on-time delivery rate than other types of first-class mail.

The new data appears to show that slowdowns in ballot delivery are nevertheless tethered to poor performance in the mail. In Detroit on Tuesday, 44.1% of first-class mail arrived on time and 51.65% of ballots arrived on time, meaning those ballots moved faster, but barely.

Again, there are significant limits to what this new USPS data can show. The numbers are based on ballots that have a special USPS tracking code, which are more likely to be used by well-funded election divisions or those with a lengthy history of running large vote-by-mail campaigns. In many districts, according to the USPS , the amount of data is too small to be statistically valid.

The data only captures processing times, meaning it doesnt capture any delays that happen at the very beginning of a ballots journey when its on a truck or in a mail carriers bag. And many districts are not processing ballots alongside the general mail at all, but rather delivering them directly from local facilities to elections officials which is faster but not reflected in these numbers.

One thing the data is not is an indicator of where it is still safe to vote by mail. Even in places with reliable delivery service, it is now too late to mail a ballot and expect it to arrive by Election Day, the due date in a majority of states.

Voters who have not voted yet should plan to do so in person, to return their absentee ballots to authorized dropboxes in states that have them, or to bring absentee ballots to local election offices in person.

Given that its the Friday before Election Day, were advising voters not to place their ballots [in] the mail at this point, said Marian Schneider, a consultant on elections and voting rights for the ACLU of Pennsylvania. But I caution against creating a panic. Voters still have a lot of options.