The bureau is hoping to get at least 67 percent of the letters mailed back, Davis said. Census takers will knock on doors throughout the country during the summer to track down the rest. For every 1 percent increase in census forms mailed back, the bureau saves $80 million-$90 million.
The results, which will become available in early 2011, will impact state and local political power - and budgets.
Cities like Philadelphia haven't done well in past counts, according to Tom Ginsberg, author of the recent report, "Preparing for the 2010 Census: How Philadelphia and Other Cities Are Struggling and Why It Matters."
In 2000, Philadelphians returned 53 percent of the forms mailed to them even though the state response rate was higher than the national average. After the census was completed, the bureau set the city's population at 1,517,550.
According to Ginsberg's report, that likely undercounted the city's population by about 8,000 people.
The city has also challenged follow-up annual surveys, arguing that the city's population is almost 100,000 residents higher than the census bureau's 2008 estimate of 1,447,395.
The census will also determine legislative apportionment at the congressional, state and local level.
And it doesn't look good for Philly or Pennsylvania.
"Pennsylvania is probably going to lose a couple congressional seats [because] Pennsylvania is a slow-growth state, and others are growing [faster]," Ginsberg said. "They are going to eclipse Pennsylvania no matter what it does."
"In the allocation of seats among the states, if there's an undercount, the state might lose a seat we don't have to," said Jack Nagel, a political science professor at the University of Pennsylvania.
In the 1920s, Pennsylvania had 36 seats in Congress. Now, it has half that number.
That means less clout in the House of Representatives during the everyday Congressional machinations, and less clout every four years during presidential elections.
Still, because of the winner-take-all rule in presidential elections, Pennsylvania is "still a big player," Nagel said. "We are still one of the pivotal states."
The redistricting that followed the 2000 census shifted a seat from low-growth western Pennsylvania to eastern Pennsylvania and pushed a couple of Philly seats out to the fast-growing Philadelphia suburbs.
In the 2010 Census, "if turnout is actually growing, it would have a huge impact on that redistricting . . . but [the Census] would have to show [Philly's population] is really growing and at a faster clip than the suburbs to cause a shift in the redistricting," Ginsberg said.
The census' effect would have a more noticeable effect on the city's power in the state government if there was a substantial undercount, Nagel said.
"An undercount would hurt Philly's ability to forestall a lack of political representation, mostly in Harrisburg [where the city could lose seats in the state House and Senate]" Ginsberg said, citing the growth of the suburbs around Philly.
But at the end of the day, the most important element of the census is that it might prove that after a half century of population decline, Philadelphia has finally started to grow again.
"When you're shrinking, that number carries more meaning because its a painful thing to see - it is against human nature to see your population shrink," Ginsberg said.
"If a city can show it's not shrinking, there are benefits in marketability and self-image, attitude, and prestige, beyond the legislative and financial."