Thanks to the Library of Congress’ American Memory project (homepage http://memory.loc.gov ), we have the entire series of published Congressional debates online. The antebellum series, under the title The Congressional Globe, is available at the link below. Unless otherwise stated, all my references are to page numbers in the Globe, 31st congress, 1st Session, searchable at the following webpage:
http://memory.loc.gov/ammem/amlaw/lwcglink.html#anchor31

In the midst of an acrimonious exchange on the Senate floor between William H. Seward of New York and William R. King of Alabama, over whether the 1850 census data could show the quality of life of slaves in the South, Thomas J. Rusk of Texas complained to Seward, “It is to be regretted, sir, that we cannot do anything here without having this interminable question brought up. It must, it seems, be brought forward when we have only the census bill up for discussion.” (page 675)

Rusk should not have been surprised. The first thing you notice about the debates in Congress in the 1840s and 1850s is how often the topic of slavery appeared. It permeated virtually every discussion as a few vehemently anti- and pro-slavery Congressmen used every opportunity to agitate the issue in their respective houses. Each session saw a pile of petitions from hundreds of Northern citizens for the abolition or restriction of slavery (see, on page 814, the 37 anti-slavery petitions introduced on 24 April 1850)– all of which were routinely tabled, but which, nevertheless, fanned the embers of hostility between Northern and Southern states. The 1850 sessions were particularly fiery on the subject of a fugitive slave bill which was eventually signed into law 18 September 1850. Arguments, which almost tore the nation apart over establishing slavery in western territories and new states, would be temporarily settled by the Compromise of 1850. In this environment, anyone who had expected the 1850 census–which necessarily included a count of the slaves–to be simply a rational enumeration of the nation’s population would be quickly disabused of that idea as opposing teams of Northern and Southern congressmen made the census their political football.

Congressmen used existing myths and fabricated new ones to suit their own agendas. For example, the speakers for the deep South portrayed slavery only as big plantations with hundreds of slaves personally unknown to their owners, while representatives of slaveholding Border States suggested a different vision of the slave owners as yeoman farmers who worked and lived with slaves whom they knew personally. Southern senators even argued that enslaved mothers were unable to know how many children they had borne.

Neither northern abolitionists nor southern slaveocrats considered simply gathering exactly the same data for free persons and slaves. Some abolitionists were bent on using census data to poke slaveholders in the eye on subjects like slaves’ health and longevity (as explored through average ages and child mortality), the migration patterns imposed by the internal slave trade, the illegal importation of African slaves (as would be suggested by a place of birth in Africa after 1808), and the results–and future implications–of racial amalgamation. One Southern gentleman wanted to test a pet theory that mulattoes were less fertile than blacks of “pure blood.” Other Southerners reacted defensively to prevent any inquiry that would undermine their message that slaves were property, or would reveal slaves to be real persons with families or social structure of any kind. Both sides, embroiled in debate over the bill that would become the Fugitive Slave Act, were interested in knowing how many runaway slaves might fall under the provisions of the act. The result was the 1850 census we know today (and its virtual clone in 1860), in which the millions of slaves are presented as nameless and faceless numbers.

Debates in the senate on 4 February 1850 (page 282) began inauspiciously for the bill “for the taking of the seventh census of the United States.” The bill contained a set of forms and schedules prepared by a committee of senators, but Mr. Dawson of Georgia pointed out that a law of 3 March 1849 had established a Census Board, charged with preparing and printing forms and schedules for taking the 1850 census. Senators Davis of Massachusetts and Underwood of Kentucky protested they were unaware that the Census Board had produced any forms, but Dawson charged that not only did these forms exist, they had already been printed, expending most of the $10,000 budget appropriated for the Census paperwork. The next few days of debate concerned how to save money by using less paper and who should be awarded the lucrative printing contracts. In March, the Senate was still debating printing costs, the method of paying enumerators, and whether “Beeswax and honey” should be stricken from the agricultural schedule! Confusion over the mandate of the Census Board had opened the door for Congress to micro-manage the content of each column in each schedule form. With all the wrangling over the cost of paper, it was not until April that debate finally touched the manner of taking the census of slaves.