More than 120 million U.S. census forms begin arriving Monday in mailboxes around the country.

The government's once-a-decade population count will be used to divvy up congressional seats and more than $400 billion in federal aid. Fast-growing states in the South and the West could stand to lose the most because of lower-than-average mail participation rates in 2000 and higher shares of Hispanics and young adults, who are among the least likely to mail in their forms.

The Census Bureau is urging cities and states to promote the census and improve upon rates in 2000, when about 72 percent of U.S. households returned their forms. If everyone who receives a census form mailed it back, the government would save an estimated $1.5 billion in follow-up visits.