Just a few relevant quotes. Read the whole thing.
"When the US Government rounded up Japanese-Americans in 1942, they used the "supposedly private" census data to tell the soldiers how many Japanese lived on each block. The Census Bureau handed out the data needed to put them into prison camps or otherwise be harassed. Reams of information came from the "strictly confidential" census. In 1943, a direct tabulation of "Every Japanese person living in Washington, DC", including name, address, sex, age, marital status, citizenship, profession, and employer, all taken directly from individual census records, was provided to the Secret Service. Throughout the war, individuals "of interest" to the FBI and Secret Service were looked up, and their private information was released for purposes of government harassment."
"The Constitution authorizes them to count heads every ten years, not to ask how many bathrooms you have and what racial group your ancestors are from."
"YOUR ethnic, religious, recreational, or occupational group might well be the next target of state-sponsored terror tactics. I'm talking about the US Government here, though of course the same information is available to both private organizations such as the KKK and the Hollywood MPAA anti-piracy enforcers, as well as to other governments. DON'T help idiotic extremists lock you up, smash your windows, terrorize your kids, sue you for having MP3s, burn crosses on your lawn, deport you for having parents from the wrong country, or draft you because you have a relatively rare skill. You can't predict what innocent trait of yours will inflame extremists next year or in the next fifty years. The best defense is to TELL THE CENSUS TAKERS NOTHING"