The first detailed results of the 2020 census, scheduled for release Thursday, are expected to show a nation that is aging, diversifying and continuing to grow faster in the South and Southwest.

The decennial census data might also show that the total white population shrank for the first time in U.S. history, with all of the nation’s growth attributed to nonwhite residents.

Demographers will be watching to see whether the U.S. crossed several other benchmarks, including the possibility that the white population dropped below 60% for the first time and whether white people will fall below 50% of the population that is younger than 18 years old.

The census also has been expected to document the continued depopulation of rural areas as cities and their suburbs grow and gain political sway.

The first census results—consisting only of population totals for states and the nation—were released in late April. The U.S. population reached 331 million last year, growing more slowly over the last decade than at any time since the 1930s.

Delayed more than four months by the Covid-19 pandemic, the new results detail the racial makeup of every county, city, neighborhood and block in the country. Legislators and commissions will immediately use them to begin redrawing local and federal voting districts in time for next year’s elections.

In addition to intrastate shifts, 13 states are set to gain or lose seats in the House of Representatives next year through the once-a-decade reapportionment required by the Constitution. They’ll also lose or gain votes in the Electoral College beginning in the 2024 presidential election.

Texas will gain two House seats and five states will gain one each: Colorado, Florida, Montana, North Carolina and Oregon. Seven states will lose one each: California, Illinois, Michigan, New York, Ohio, Pennsylvania and West Virginia.

The new results will become the benchmark for everything from disease and death rates to government-funding formulas and market research. They will also fuel research on issues ranging from segregation and gentrification to suburban sprawl. More results, including detailed age and racial breakdowns as well as family relationships and homeownership rates, also have been delayed and aren’t yet scheduled for release.

The census has been dogged by problems and disputes that began even before data collection started. In 2018, Trump administration Commerce Secretary Wilbur Ross ordered a citizenship question added, claiming it was necessary to provide data to enforce the Voting Rights Act. Civil-rights groups fought that effort in court, saying it was an attempt to discourage immigrants from participating in the census.

The Supreme Court in June 2019 blocked such a question from being added, citing evidence that Mr. Ross’s justification was “contrived.”

In March 2020, millions of Americans had just received a mailed invitation to respond to the census when the pandemic mushroomed. That forced the bureau to shut down many operations for weeks. When it restarted, officials struggled to retain and deploy a quarter-million mask-protected workers to count millions of non-responding households, many of them wary of answering the door. The bureau planned to extend counting last fall, but Mr. Ross ordered it accelerated. That produced more litigation before the Supreme Court allowed counting to close Oct. 15.

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In the end, about two-thirds of Americans answered the census on their own, a key measure because that method typically yields the most complete, accurate results. That matched the 2010 rate, although last year the window was extended from 20 to 31 weeks. Other preliminary quality measures show a sharp increase in the number of census questions left blank. The bureau has filled in blanks by checking other federal records and making inferences from information about other household members or, as a last resort, similar households nearby.

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The bureau also has faced criticism for a new plan that adds small, calculated distortions to most census results to protect individual respondents’ information. Testing has shown effects would be minimal in most instances but potentially larger when it comes to some smaller groups and areas. The plan, which has undergone more than two years of development and testing, has drawn concerns from some data users that census results won’t be usable for various needs, such as for research and distributing state and federal funds.

Amid skepticism about the quality of the census, the bureau has taken extra steps to assess its accuracy. After sifting a trove of operational data, a panel from the American Statistical Association is expected to report early findings within weeks. The bureau has also asked for a full review by a panel of the National Academy of Sciences, a project expected to last into 2023.

Even before the disruptions, the census schedule included two large, independent cross-checks. The first compares it to data on births, deaths and immigration. Early results show that the overall count fell near the middle of those demographic estimates.

The second is a survey of 161,000 households that is matched to completed census forms. By finding survey respondents who didn’t fill out the census form, the bureau can estimate how many and what types of people were missed. Results are expected early next year.

Despite the range of problems, there is no plan to readminister the 2020 census. The bureau will accept local governments’ documented challenges but only to correct processing or geographical errors, such as missing a housing unit or counting it in the wrong place.

—Chad Day and Anthony DeBarros contributed to this article

Write to Paul Overberg at Paul.Overberg@wsj.com and John McCormick at mccormick.john@wsj.com