Updated at 12:18 p.m. ET Saturday

Under pressure from the Trump administration to deliver 2020 census results by the end of this year, the U.S. Census Bureau has set a cutoff date for receiving paper forms for the once-a-decade head count, NPR has learned.

The bureau confirms to NPR that it plans to only process paper census questionnaires postmarked by Sept. 30 — its new end date for all counting efforts — and received by Oct. 7 at one of its two data processing centers in Phoenix and Jeffersonville, Ind.

Although the vast majority of households (80%) that have filled out a census form on their own did so online, paper forms have been the second-most popular way for those households — about 1 in 5 — to get counted, especially in rural areas.

"Based on historical experience and the very long self-response period in 2020, we expect very few mail returns that late in the process," the bureau said Friday in a statement, which also noted that it has not experienced delays in receiving completed census forms this month or in July.

Still, recent cuts to the U.S. Postal Service are raising concerns among census advocates. While much of the country's attention has been on the potential impact on November's election, some census watchers are worried that any delivery delays over the next few weeks could jeopardize the accuracy of census data collected from sparsely populated parts of the country.

That, in turn, could have lasting consequences on the share of federal funding and political representation tied to census numbers that residents in those areas receive for the next decade.

Citing NPR's reporting, Sen. Kamala Harris of California, the Democratic vice presidential candidate who is also a member of the Senate Homeland Security and Governmental Affairs Committee, has submitted written questions to Postmaster General Louis DeJoy, who testified before the committee on Friday.

Harris is asking whether DeJoy will "guarantee" that the Postal Service will deliver census forms postmarked by Sept. 30 to a Census Bureau data processing center by Oct. 7.

Asked by NPR if the Postal Service would make that guarantee, spokesperson David Partenheimer provided a statement that did not directly answer the question, saying instead that the service "has more than enough capacity, including collection boxes and processing equipment, to handle Census mailings the rest of this year."

Earlier this year, the Postal Service delivered around 590 million pieces of mail nationwide for the 2020 census — a job the service described as the "single largest First-Class Mail mailing within a 90-day period in USPS history."

"These paper forms that get sent to rural communities are really the lifeline to those rural communities participating in the census," says Rebecca DeHart, CEO of Fair Count, an Atlanta-based nonprofit organization that's been trying to boost census response rates in some of the most remote parts of Georgia.

Fair Count started installing Wi-Fi routers, portable hot spots and laptops last year in churches and community centers to try to prepare rural communities in the state for the first primarily online U.S. census. But DeHart says the group's community organizers have found that some residents prefer a more traditional way of taking part in the constitutionally mandated count of every person living in the country.

"People feel comfortable with a paper form," DeHart says. "They're able to read it. They're able to see it for themselves."

Among some households in rural communities that don't have regular mail delivery and in certain areas recovering from major disasters, close to half that have responded so far have done so by mailing back a paper form that was hand-delivered by a Census Bureau worker. But because COVID-19 and stay-at-home orders forced the bureau to keep its staff out of the field earlier this year, many of those residents waited for months to have a worker hang a plastic bag with a census form outside their homes.

Households that do get mail regularly were supposed to receive at least one physical questionnaire if they didn't participate in the census by April.

But Johnnie Mae Adams, 73, says she has been waiting for months to receive a paper form in Millen, Ga., a small town about an hour south of Augusta in eastern Georgia where less than a third of households have self-responded to the census.

"For a lot of people, mail is always late," says Adams, a retired educator who moved back to her hometown to help care for one of her aunts.

Adams says she's only seen a Census Bureau postcard reminding her to participate in the 2020 count, and as of Friday, none of the bureau's door knockers had stopped by yet to do an in-person interview.

"I understand the county's funding comes according to the census, and everybody needs to be counted," Adams adds.

Adams could try to do the census online using her smartphone, but Internet service, she says, is often spotty. She's thought about calling one of the toll-free numbers for the bureau's call centers, but having a paper form, Adams says, would make it easier for her, her aunt and other relatives living in the same home to answer all of the questions together.

As of Tuesday, the Census Bureau says it has counted 71.6% of homes across the country. Its workers now have less than six weeks to try to reach tens of millions of homes before counting is set to end on Sept. 30.

Last month, Census Bureau Director Steven Dillingham made a surprise announcement, saying the bureau would send out an additional paper form to try to increase census self-response rates.

That mailing is expected to arrive starting Saturday and through Sept. 15, the bureau tells NPR. On Friday, the bureau confirmed it's mailing 16.2 million questionnaires to households that have not been counted yet in the more than 71,000 census tracts that had a self‑response rate of 65% or lower on July 28. Unresponsive households in some rural areas that should have received two forms by now are not expected to get another questionnaire soon.

For months, some census advocates, including Arturo Vargas, CEO of the National Association of Latino Elected and Appointed Officials Educational Fund, have been urging the bureau to mail out another paper form.

"Based on our research, Latinos express a preference for paper because they have more confidence that their information would be received by the Census Bureau rather than just being submitted online," Vargas says about findings from surveys and focus groups that NALEO has commissioned over the past two years.

But Vargas says the new Sept. 30 end date for all counting efforts "almost defeats the purpose" of another mailing.

On Tuesday, the National Urban League, three cities in California and other groups filed a federal lawsuit against the Trump administration to try to get counting extended through Oct. 31, as the Census Bureau had previously planned, in response to delays caused by the coronavirus pandemic.

Attorneys with the Mexican American Legal Defense and Educational Fund and Asian Americans Advancing Justice - AAJC are also asking another federal judge to take action in an expanded federal lawsuit in Maryland.

Dillingham, the bureau's director, however, has said Commerce Secretary Wilbur Ross, who oversees the bureau, has directed it to finish counting soon in order to deliver the first batch of census results to the president by the end of this year, contradicting top career officials who have said the pandemic has made that no longer possible.

Both of the lawsuits are under a time crunch to try to get a court ruling that throws out the shortened census schedule before the bureau wraps up counting.

Terri Ann Lowenthal, a former staff director of the House oversight subcommittee for the census who now consults on census issues, says she supports the bureau's decision to mail another paper form to unresponsive households "given the steep hill the Census Bureau must still climb to finish counting operations on time."

"However, the compressed schedule now creates a confluence of factors that could make reliance on that form problematic," Lowenthal says in an email. "With the Postal Service warning of mail delays in the fall, forms returned by mail could arrive too late to be processed, leaving the Bureau to rely on household data collected or produced from a less direct and, therefore, less accurate source."

Equipped with face masks and bottles of hand sanitizer, door knockers from the bureau are trying to conduct socially distanced interviews with households that have not yet been counted. If that's not possible by the third try, census workers are trained to try to figure out who's living in those homes by interviewing neighbors instead. As another alternative, the bureau may turn to government records to try to fill in the gaps of missing information.

But Lowenthal warns that people are more likely to be left out of the census when they're counted through these efforts rather than on their own.

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