If government estimates hold true, the closely watched 2010 census will show America's once-torrid population growth dropping to its lowest level in seven decades. In Congress, the steady migration to the South and West should be a boon for Republicans, with GOP-leaning states led by Texas picking up House seats.

The Census Bureau expects to release Tuesday the first results from the once-a-decade government count, figures that will be used to reapportion the 435 House seats among the 50 states. The numbers trigger a high-stakes process wherein the dominant party in each state redraws the election map, shaping the political landscape for the next 10 years.

Census estimates provided this month based on birth and death records place the 2010 count somewhere between 305.7 million and 312.7 million, up from 281.4 million in 2000. That range means U.S. growth over the previous decade would be at a slower pace than the 13.2 percent increase from 1990 to 2000...