Commerce Secretary Wilbur Ross plans to ask Congress for a major increase in funding Thursday to salvage the 2020 Census, a program beset by cost overruns, poor preparation and a population of Americans who are less likely than at any point in recent history to self-report their existence to the federal government.

The Commerce Department now estimates that the decennial effort will cost $15.6 billion — $3.3 billion, or nearly 27 percent, more than earlier estimates by the Census Bureau, according to a document obtained by The Washington Post.

The testimony before the House Committee on Oversight and Government Reform on Thursday will put Ross, a longtime champion of the census, at the center of a potentially explosive political fight about spending for the count, which has faced criticism in recent months for being dangerously underfunded.

The results of the constitutionally mandated survey determine the distribution of congressional seats, affect the shape of districts and help decide the flow of more than $675 billion a year in federal funding. The full count of all people living in the United States and its territories every 10 years is also vital to businesses, civic organizations and others whose work relies on demographic data.

The request comes amid partisan wrangling over how congressional districts are drawn within the states, and legal challenges to gerrymandering that affects minority representation in Congress. An inaccurate decennial count has become a concern for leaders of both parties, as has fear that the 2020 count will be targeted by cyberattacks.

Historically, populations that are more likely to support Democratic politicians — including the poor, the transient, minorities and immigrants — are less likely to respond to the census, leading to undercounts. The 2020 count approaches at a time of heightened fears about deportation of undocumented immigrants and bans on people from some countries entering the United States, raising concerns that typically hard-to-count populations may be even more reluctant than usual to share information with the government.

But this can be self-defeating. "Not being counted means not being heard and not being served, because the census drives billions in funding and it changes how we are represented in Congress," said Meghan Maury, the policy director of the National LGBTQ Task Force, who is a member of the Census Bureau's National Advisory Committee on Racial, Ethnic and Other Populations.

Many Republican leaders, including those who will be responsible for overseeing and funding the 2020 Census, have also expressed concern about ensuring a successful count. But there are clear differences between the parties when it comes to funding early planning work, including advertising and outreach aimed at minority groups.

At the hearing, Ross is expected to warn of declining national interest in participating in the census. In 2010, 63.5 percent of people voluntarily responded to the initial mailing. In the 1970s and 1980s, initial response was more than 75 percent.

The projected self-response rate for 2020 has fallen to between 55 percent and 60.5 percent, according to the document. The reasons include distrust of the government, concern about using the Internet to provide information, and a more diverse and mobile nation with a higher number of non-English-speaking households than in the past.

Asked about the document, Commerce spokesman James Rockas said, "As a former enumerator himself, Secretary Ross is keenly aware of the unique challenges posed by the census." He added that Ross is looking forward to giving "a full accounting of those needs to Congress and the public."

Typically, in the years leading up to the decennial count, funding is ramped up significantly. But in recent budgets, funding flatlined, sparking predictions that to get an accurate count the bureau would later have to play catch-up, costing more in the long run.

Those predictions appear to have come true, said John Thompson, who resigned in May as director of the bureau. Congressional allocations for 2017 and previous projections for 2018 fell significantly short of what the bureau requested, and the lost time may not easily be made up.

"Had we been funded to do everything we asked for, then we'd be much farther ahead," Thompson told The Washington Post. "At this point, they're going to have to go back and do some of it the old way . . . with paper and pencil. That's not bad in terms of accuracy, but it does mean it's going to cost more."

It is not clear whether Ross will request additional 2018 funding for the entire bureau, which also administers other surveys such as the Economic Census and the American Community Survey.

Doing things the old way is just what bureau officials had sought to avoid when planning the 2020 count. Inflation and a larger population meant that conducting a count identical to the 2010 Census would have cost $5 billion more in 2020. By relying on new technology, including online responses, mapping software and novel uses of public data, the bureau said it could reduce the number of workers knocking on doors and keep the 2020 cost just over $12 billion, only slightly above the 2010 cost.

But Congress declined to fully fund the census program, even at these lower levels. Between fiscal 2012 and 2017, the entire Census Bureau budget has come in about 10 percent below the amount requested. The bureau declined to comment for this article.

In response to the growing concern, Ross announced this summer a top-to-bottom review of the program. His new proposal includes $1.2 billion in funding for expected cost overruns.

Ross's review found several problems with the original projections, concluding that contracts were not properly managed and projected savings were not realistic. The Commerce Department now projects that higher wage rates for workers in 2020 will raise costs, as will a need for contingency planning to respond to unexpected events, including a possible cyberattack on the new online census program.

It is unclear how much of the additional funding would be used to make up for the reduced planning and preparation over the past several years. Census preparation that was once deemed essential — including new Internet response testing, earlier advertising planning and evaluation of data software — has been canceled or delayed in the face of congressional spending limits.

Ironically, the shortage of money for planning has stymied measures designed to cut costs.

In 2014, for example, the Census Bureau announced it could save $900 million by using computer imagery and third-party data to build a database of every address in the nation, a process that had been accomplished in 2010 by hiring 150,000 field workers.

But because of budget cuts, the bureau has faced delays in crucial tests to the new system, according to a June report by the Government Accountability Office. Tests conducted in 2016 had not been evaluated earlier this summer when the bureau announced that the next round of testing, which was intended to address problems from the previous tests, would be canceled.

Census officials told auditors that the lack of testing would result in sending field workers to 30 percent of the nation's addresses, up from 25 percent in previous plans. The bureau has not released specific estimates for those new costs.

In 2017, the bureau also canceled field tests in Puerto Rico, North Dakota, South Dakota and the state of Washington in response to budget pressures. And after President Trump released his 2018 budget, the bureau announced it would also cancel two of three "end-to-end" field tests scheduled for next year, including another site in Washington and one in West Virginia.

Budget cuts also have forced the Census Bureau to cut back on planning for the communications program. Proposed tests have been withdrawn and a poll of 35,000 Americans to determine attitudes toward the census planned for late 2017 has been delayed. Other programs designed to motivate hard-to-reach groups also have been pushed back or canceled.

"We were expecting to have more right now, and we don't," Maria Olmedo Malagon, the program manager for the 2020 Census communications plan, said at an April public meeting. "There are definitely several activities that we would like to be tackling right now, but we can't."

Delays have also plagued the census partnership program, which focuses on outreach to large corporations and community groups. Currently, only 40 new employees are focusing on working with clergy members, corporations and other community leaders who can help drive home the importance of being counted. By 2020, there should be 4,000 to 6,000 partner specialists in place, Thompson said, adding that by this point in the cycle the bureau should ideally have hired between 100 and 200 people.

"If you want to talk to McDonald's about getting signage into every store, you have to be talking to their chief marketing officer now," said Steve Jost, a former census official who oversaw the 2000 and 2010 outreach campaigns. "If you want in-store promotions at Target and Walmart, all those things take lead time, and they are all at risk now."

Advocacy groups are also concerned about the slow start. The NAACP filed a lawsuit against the Commerce Department last week, saying it had not disclosed information about plans for the upcoming count and warning that a lack of transparency could end up harming minorities.

Historically, minorities are counted less accurately than whites. A census study of the accuracy of the 2010 survey found that there was a net overcount of just 0.01 percent overall, driven by an 0.8 percent overcount of non-Hispanic white residents. But the undercounts among other populations were much higher: 2.1 percent for blacks, 1.5 percent for Hispanics and 1.1 percent for renters.

The 2020 Census will rely heavily on online responses, which would be the first shift in basic technology since 1970, when the bureau began using the postal service to collect household population counts.

A recent effort to improve technology was disastrous. In the run-up to 2010, the bureau attempted to introduce wireless handheld devices to collect information from people who didn't mail in forms, but problems with the devices and a lack of adequate testing forced officials to revert to pen and paper. The census aims to deploy new mobile technologies in 2020.

Even if Congress approves Ross's proposed new funding, it might not be enough, said Rep. Elijah E. Cummings (Md.), the ranking Democrat on the House oversight committee. "When you starve the census of funds every single year, pretend inflation suddenly stopped a decade ago, and force the bureau to cut critical tests . . . then you can't act surprised when estimates do not hold and savings do not materialize," he told The Post. "I am very concerned that even these new requests are far below what is needed to hold a fair and accurate census as the Constitution requires."

Rep. Carolyn B. Maloney (D-N.Y.) plans to introduce a bill on Wednesday that would increase 2018 funding for the Census Bureau by $441 million more than the initial Trump budget, or more than $200 million more than Ross proposed for the decennial count.

At the same time, some Republicans have focused on cost overruns to argue against pouring more money into the effort.

Rep. Trey Gowdy (R-S.C.), who chairs the House oversight panel where Ross will testify, said time is running out for the bureau to address problems with the census program. "The stakes are too high to get this one wrong," he said in a statement.

In the past, conservative-led efforts have sought to curtail other bureau activities such as the annual American Community Survey.

But other Republicans acknowledge the importance of an accurate count. Alabama Gov. Kay Ivey has been touring her state for weeks telling government officials and business leaders that she's "dead serious" about ensuring everyone fills out a form.

"Alabama is in jeopardy of losing a congressperson," she told the Association of County Commissions of Alabama in August. "But more devastating than that could be the fact that we will lose federal funding."

After the 2000 Census, North Carolina ended up winning an extremely close battle with Utah for the final congressional seat. North Carolina won the seat by a margin of just 856 more people.

Heather Long contributed to this report.