(AP) — The Census Bureau plans to announce it will miss a year-end deadline for handing in numbers used for divvying up congressional seats. That delay could undermine President Donald Trump’s efforts to exclude people in the country illegally from the count if the figures aren’t turned in before President-elect Joe Biden takes office.

It will be the first time that the Dec. 31 target date is missed since the deadline was implemented more than four decades ago by Congress.

Internal documents obtained earlier this month by a House committee show that Census Bureau officials don’t see the apportionment numbers being ready until days after Biden is inaugurated on Jan. 20.

Once in office, Biden could rescind Trump’s presidential memorandum directing the Census Bureau to exclude people in the country illegally from numbers used for divvying up congressional seats among the states. An influential GOP adviser had advocated excluding them from the apportionment process to advantage Republicans and non-Hispanic whites.

“The delay suggests that the census bureau needs more time to ensure the accuracy of census numbers for all states,” said Terri Ann Lowenthal, a former congressional staffer who specializes in census issues.

By law the Commerce Department must present the president by year’s end with population figures from the 2020 census, data then used to determine how many seats in Congress each state gets. The president then is required to submit the numbers to Congress in early January. The Commerce Department oversees the Census Bureau, which conducts the once-a-decade head count of every U.S. resident.

However, there are no penalties for missing the deadline.

“For the Census Bureau, goals No. 1 , 2 and 3 are completeness, accuracy and usefulness. They like to maintain the schedule, but that can’t be a priority for them,” said Kenneth Prewitt, a former Census Bureau director during President Bill Clinton’s administration.

Besides deciding how many House seats each state gets, the census is used for determining how $1.5 trillion in federal funding is distributed each year.

Trump’s July order on apportionment was challenged in more than a half dozen lawsuits around the U.S., but the Supreme Court ruled earlier this month that any challenge was premature, allowing the plan to move forward. The Census Bureau hasn’t publicly revealed how it plans to determine who is in the country illegally since the Supreme Court last year prohibited a citizenship question from being added to the census questionnaire.

After the pandemic caused hiring shortages and prompted the Census Bureau to suspend field operations in the spring, the statistical agency asked Congress for extensions. The requests included one that would push the deadline for handing in the apportionment numbers from the end of the year to next spring.

At the time, Trump said, “This is called an act of God. This is called a situation that has to be. They have to give it.”

The request passed the Democratic-controlled House but went nowhere in the Republican-controlled Senate after Trump issued his order.

A coalition of municipalities and advocacy groups sued the Trump administration after it changed the schedule once again to shorten census field operations by a month and return to the Dec. 31 deadline for handing in the apportionment numbers. The plaintiffs argued the count was shortened by the Commerce Department so that census data-crunching happened while Trump was still in office, and they said it would cause minorities to be undercounted.

They also worried that the shortened field operations and data processing would jeopardize the count’s accuracy and completeness. Bureau statisticians have been given only half the time originally planned to crunch the numbers, and Census Bureau director Steven Dillingham said last month that agency statisticians had found anomalies in the 2020 data that have popped up in past censuses.

The Census Bureau’s watchdog agency on Wednesday said it was concerned about lapses in quality control checks meant to detect falsifications by census takers. The Office of Inspector General said the Census Bureau failed to complete 355,000 re-interviews of households to verify their information was accurate.

Even top Census Bureau officials internally questioned being able to meet the Dec. 31 deadline, with associate director Tim Olson telling colleagues in an email that anyone who thought the census numbers could be crunched by year’s end “has either a mental deficiency or a political motivation.” The email was disclosed in the litigation.

Having a deadline for turning in the apportionment numbers is less than a century old — a byproduct of a Republican-controlled Congress’ refusal to reapportion after the 1920 census, out of fears it would shift political power from rural areas to cities where immigrants and others had settled. For the first 14 censuses of the nation’s 24 head counts, the U.S. managed without a deadline, said Margo Anderson, a historian at the University of Wisconsin-Milwaukee.

“It has not been a very controversial issue until this year, the year of a pandemic and the Trump administration flip-flopping on its goals and its efforts to take the undocumented out of the apportionment count,” Anderson said. “They are still trying to wrestle the numbers into something that looks right.”