Amid a record surge in early voting, millions of mail ballots remained unreturned Wednesday, prompting a flurry of warnings from election officials that ballots sent via the U.S. Postal Service at this point may not arrive in time to be counted.

With Election Day less than a week away, more than 42 million out of the 92 million mail ballots requested by voters nationally had not yet been returned as of Wednesday afternoon, according to data from the U.S. Elections Project, a nonpartisan site tracking early voting.

In the 20 states where party affiliation data was available, more than 11 million of the outstanding ballots had been requested by Democrats, nearly 8 million by Republicans and about 10 million by unaffiliated voters, according to the Elections Project.

Tuesday marked the last day that postal and election administration experts said ballots could be mailed to ensure delivery in time for Election Day, based on national first-class mail service.

The passing of the informal deadline set off a flurry of advisories from election officials and posts on social media encouraging voters who had yet to postmark their ballots to instead drop them off at voting centers — or consider voting in person.

Early-voting figures across the country continued to soar, with more than 75 million Americans casting ballots as of Wednesday. The number of voters who have participated so far in this year’s election has already surpassed half of the total turnout from 2016.

Election officials said many of the outstanding mail ballots may belong to voters who requested them early in the process, then changed their minds and decided to vote in person — although it’s impossible to determine how many.

In North Carolina, about 600,000 ballots remained outstanding out of 1.4 million requested, according to Damon Circosta, the Democratic chairman of the North Carolina State Board of Elections.

“Some of those voters may have chosen already to go and vote early in person; some may vote on Election Day,” he said. “But there’s still a significant number of outstanding absentee ballots, and we’re asking them to mail them or drop them off as soon as possible.”

The numbers were similar in Georgia, where roughly half a million ballots had yet to be returned, of about 1.5 million requested. Georgia Deputy Secretary of State Jordan Fuchs said voters may have lost confidence in voting by mail after originally seeking absentee ballots.

“You have a handful of candidates who are saying you can’t trust it,” Fuchs said, “so you’re seeing a response from those who intended to vote absentee and are now going to vote in person.”

Voters across the country have also expressed concerns that mail delays could impede the delivery of their ballots.

Timely mail service fell to distressingly low levels a week out from the presidential election, according to new data from the U.S. Postal Service, with on-time delivery rates in crucial battleground states falling below 60 percent.

In the Philadelphia Metro postal district — which includes the city and suburbs crucial to Democratic nominee Joe Biden’s hopes in the state — 43 percent of Tuesday’s mail was delivered on time, the Postal Service reported to the District Court of the District of Columbia in filings related to litigation over the agency’s service. That represented a steep drop from earlier in the week: 61.2 percent on Monday and 58.4 percent on Saturday.

In Detroit, 52.6 percent of Tuesday’s mail was on time. In Northern Ohio, 56 percent; in Wisconsin, 60.7 percent; in South Florida, 62.9 percent.

In reporting the data, Justice Department lawyers representing the Postal Service cautioned that the agency does not consider daily data reliable. Individual days of the week sometimes follow patterns of higher or lower service, Justice Department attorney John Robinson wrote in a court filing. The data also does not include “first mile” and “last mile” handling steps, which could add time to delivery.

The Postal Service reported that its delivery of ballots was faster than other mail. On Tuesday, the agency delivered 95.1 percent of completed ballots from voters to election officials on time and 89 percent of requested ballots from election officials to voters. However, the Justice Department lawyers said that those numbers were also unreliable and that the Postal Service preferred to submit weekly data, rather than daily, as District Judge Emmet G. Sullivan had ordered.

The Postal Service’s ballot delivery figures, some critics say, do not fully depict its ballot-handling performance. Those rates apply only to mail the Postal Service was able to identify as ballots.

Postal Service leaders say that moving election mail is the agency’s top priority and that they have mobilized extra resources to respond to ballot volumes.

Post offices are taking “extraordinary measures” to move election mail, according to an internal memo obtained by The Washington Post, including creating ballot-only lines at customer service windows and drive-through ballot drop-off areas. Processing facilities are authorized to sort and postmark ballots at local post offices rather than sending them to regional processing plants, the memo states.

The uneven mail delivery comes as the agency is under overlapping federal court orders to pour resources into timely delivery ahead of the election. Before Postmaster General Louis DeJoy, a major donor to President Trump’s campaign and a former supply-chain logistics executive, took office in June, the Postal Service routinely delivered upward of 90 percent of first-class mail on time.

After DeJoy implemented cost-cutting initiatives in July, performance scores plummeted, then briefly rebounded at the end of the summer. But the agency appears to have given back almost all of those gains — only 69.8 percent of mail was on time nationally Tuesday — and then some, especially in some of the most electorally significant areas.

In 17 postal districts representing 10 battleground states and 151 electoral votes, the average on-time delivery rate for first-class mail was 64.2 percent on Tuesday, 27.6 percent lower than the performance score for those districts at the beginning of the year.

Sullivan on Tuesday ordered Postal Service executives to formally rescind DeJoy’s directives, including a stricter transportation schedule that banned late and extra dispatch and delivery trips. He also ordered daily hearings with attorneys from the Justice Department and voting rights advocacy group Vote Forward to monitor the Postal Service’s performance.

Every day, processing facilities are mandated by court order to conduct “all clear” checks, or sweeps of sorting plants to detect misplaced election mail. But in 10 percent of the reports, the Postal Service either found ballots that should have been processed, did not complete the check or did not report the results, according to records filed in multiple lawsuits.

People familiar with the lawsuits against the Postal Service, who spoke on the condition of anonymity to discuss ongoing negotiations, said they want the agency to deliver more than 97 percent of ballots in both categories on time.

“It’s not optimal,” said Tammy Patrick, a senior adviser at the government watchdog group Democracy Fund and the former federal compliance officer at the Maricopa County, Ariz., Elections Department. “It’s definitely demonstrated there’s room for improvement.”

The Postal Service has processed more than 523 million election mail items — ballots, ballot applications and official voter information — up from 323 million from the 2016 general election cycle, the agency reported.

The country has seen unprecedented levels of early voting, with broad swaths of the electorate mobilized by the coronavirus pandemic and a bitterly contested presidential race to exercise their franchise ahead of Nov. 3. If the trends continue, a majority of votes will have been cast before Election Day for the first time, and overall participation could reach levels not seen since the early 20th century.

In several battleground states, early voting was close to surpassing 2016 turnout, according to data from The Post and the Elections Project. In Texas, more than 8 million people have cast ballots already, representing 91 percent of all votes in 2016. The figure was 77 percent in Georgia, 76 percent in North Carolina, 72 percent in Florida and 68 percent in Arizona.

Early voting was less robust in key Midwestern and Rust Belt states. In Wisconsin, early votes accounted for 52 percent of all ballots cast in 2016; in Michigan, 46 percent; in Ohio, 42 percent; and in Pennsylvania, 32 percent. But in most of these states, early votes have already far exceeded the final 2016 early votes.

“We’ve never even seen close to this number of early voters,” said David Becker, executive director of the nonprofit, nonpartisan Center for Election Innovation and Research. “I think what that’s telling us is that for all of the partisan gamesmanship, for all of the last-minute litigation, for all of the legitimate concerns we all have about the pandemic and foreign interference, the voters are saving us. The voters have gotten the message to make a plan and to vote early. And they’re working hard to educate themselves about their options, and they’re making their voice heard.”

Aaron Blake contributed to this report.

Correction: An earlier version of this story erroneously attributed to Georgia Deputy Secretary of State Jordan Fuchs the idea that President Trump’s attacks on mail voting had deterred voters from casting their ballots through the mail.