“We remain deeply troubled by the persistent and disproportionate undercount of our most vulnerable citizens — people of color, very young children and low-income Americans,” said Marc Morial, president of the National Urban League and chairman of the Census Bureau’s 2010 Census Advisory Committee. “At a minimum, the census should have the ability to make an adjustment in the official count to ensure that these individuals enjoy the political representation and fiscal resources to which they are entitled.”

The Supreme Court ruled in 1999 that federal law barred the use of sample surveys to adjust census results for purposes of allocating House seats; it left the door open to adjustments for other uses such as congressional redistricting or distribution of federal funds. Shortly after taking office in mid-2009, Groves ruled out statistical adjustments in 2010 for redistricting, citing a lack of preparation time.

On Tuesday, the Census Bureau noted how its efforts to count U.S. residents have improved over time. An undercount of the total U.S. population reached as high as 5.4 percent in 1940, the first time the accuracy of a census was formally measured, and then gradually decreased before an over-count was posted in 2000. American blacks are still the most likely to be missed; their undercounts have improved from a high of 8.4 percent in 1940 but at a slower pace than that of whites.