The government announced yesterday that the 1990 census put the official U.S. population at 249,632,692, a number that falls considerably below earlier Census Bureau estimates and leaves unsettled the question of the accuracy of the once-a-decade headcount.

While critics focused on a discrepancy of nearly 4 million residents between yesterday's official figures and Census Bureau projections last fall, census officials defended their numbers as accurate.

"I believe we have seen here a full, fair and efficient census," said Michael R. Darby, Commerce Department undersecretary with responsibility for the Census Bureau.

But Rep. Thomas C. Sawyer (D-Ohio), chairman of the Post Office and Civil Service subcommittee on the census, said the numbers released yesterday still should be viewed as preliminary. "I don't think we can be satisfied yet that we have the most accurate count that we are capable of producing as a nation," he said.

Population figures carry enormous weight in determining the distribution of political power and federal funds. With the House limited by law to 435 seats, each district represented about 520,000 people in 1980, when the population was 226,504,825. That now is 10 percent higher and each district will represent an average of 572,466 people.

The new numbers underline a dramatic population shift from the Midwest and Northeast to the South and West, and mean that California, Florida and Texas together will gain 14 U.S. House seats.

Virginia, whose population grew from 5,347,000 in 1980 to 6,216,568, will gain a seat in the House, for a total of 11, while Maryland's delegation will remain at eight to represent a population of 4,798,622.

The Census Bureau also announced a final count for the District of 609,909. District officials had challenged the preliminary figure of 574,844, which would have been the first time since 1930 that the city's population fell below 600,000. The District's 1980 population was 638,000.

California's gain of seven seats in the House will give it a delegation of 52 representatives, the largest any state has ever had. When new political districts are drawn over the next two years to accommodate a 26 percent population gain, a Californian will hold one in every eight House seats.

Florida, which grew by 33.4 percent, will gain four seats, for a delegation of 23, and Texas, with growth of nearly 20 percent, will add three lawmakers, giving it 30 House seats.

Experts disagree on whether the new political map will benefit Republicans or Democrats. Seven of the eight states that gain House seats voted Republican in the 1988 presidential election, leading some political analysts to predict that population shifts to those states could mean a gain for the GOP.

At the same time, many state legislatures -- including California, Texas and Florida -- are in the hands of Democrats, who will control the redrawing of congressional and state legislative districts, a process with enormous impact on political fortunes.

The figures released yesterday found 922,819 U.S. military and federal employees stationed overseas, a group that was not counted in 1980.

That addition, as well as persons counted late in the process, raised by nearly 4 million this summer's preliminary estimates of the total population. Still, the official figure fell about 3.8 million short of a Census Bureau estimate of 253.4 million released in October.

"The raw count does not compare very favorably with the 253.4 million that was the product of the most comprehensive current estimate," Sawyer said.

The gap between the figures is "enough to raise a legitimate question in terms of how accurate were the overall numbers at various stages of the process," said Kimball Brace, president of Election Data Services, a political consulting firm. The discrepancy could come from errors in the projections or an undercount in the census, he said.

Studies revealed that the 1980 census missed more than 3 million people. That undercount was disproportionately high among minorities and inner city residents, with some experts estimating that more than 20 percent of young black men were not counted.

Census Bureau Director Barbara Everitt Bryant said studies determining whether the 1990 headcount missed a substantial number of residents will be completed this spring. "In mid-'91, we will know how accurate the census was overall," she said.

But the current discrepancy between the total population figure and earlier estimates could heighten pressure to use a statistical formula to "adjust" the figures to compensate for any undercount.

Under a legal settlement between the government and several communities seeking an adjustment, including New York City, Commerce Secretary Robert A. Mosbacher must decide by July 15 whether to adjust the 1990 census in that way.

If Mosbacher chooses to adjust the numbers, it would be the first time in history that a census relied so heavily on statistical formulas and a sample survey, rather than on the person-by-person count that has been used for 200 years.

While such an adjustment would mute the criticism of big cities such as New York and Chicago, which have long argued that the census missed many of their residents, it inevitably would raise new arguments about which set of numbers was more accurate.

The figures released yesterday do not contain breakdowns for cities or counties. Nevertheless, the state totals confirm that the population has continued to shift from the nation's center to its coasts.

"The West is the big population winner," Bryant said, with growth of 22.3 percent. The South grew by 13.5 percent, while the Northeast grew by 3.4 percent and the Midwest by 1.3 percent.

Only four states and the District lost population: West Virginia, Iowa, North Dakota and Wyoming.

Overall, 19 seats in the House will shift as a result of the census, with eight states gaining and 13 states losing membership. Other winners include Arizona, Georgia, North Carolina and Washington, each gaining a seat.

New York will lose three seats. Illinois, Michigan, Ohio and Pennsylvania each will lose two. Iowa, Kansas, Kentucky, Louisiana, Massachusetts, Montana, New Jersey and West Virginia each will lose one seat.

Massachusetts came closest to retaining that seat, losing it by only 12,606 residents.

Population shifts were dramatic in some cases, with Nevada increasing its population by 50 percent, Arizona by more than 35 percent and California by 26 percent.