Actual Enumeration is a Constitutional requirement established by the Supreme Court which counts the exact number of people in a district during a census, rather than sampling or estimation. Districts in a state are required to contain roughly the same number of people, so that each person's vote will be approximately equal. When a census occurs, every state "redistricts" in order to maintain an equal number of citizens between districts.
Article I Section II of the Constitution reads: "The actual Enumeration shall be made within three years after the first meeting of the Congress of the United States, and within every subsequent term of ten years, in such manner as they shall by law direct."
The Supreme Court affirmed a Constitutional requirement of Actual Enumeration in Reynolds v. Sims 377 U.S. 533, 558 (U.S. 1964) (quoting Wesberry v. Sanders, 376 U.S. 1 (1964)) "In that case we decided that an apportionment of congressional seats which "contracts the value of some votes and expands that of others" is unconstitutional, since "the Federal Constitution intends that when qualified voters elect members of Congress each vote be given as much weight as any other vote . . . .""
Since actual enumeration is established in Article I of the U.S. Constitution, it seems to imply that census enumeration is the responsibility of the Legislative branch. Thus, President Obama's attempt to have the Executive Branch oversee his planned census is unconstitutional.
Impact on Presidential Election
The census of 2010, if done properly, could have a significant impact on the presidential election of 2012, as a shift in electoral college votes from "blue" (Democratic) states to "red" (Republican) states is expected. Texas, for example, may pick up three new electoral college votes.