In Dred Scott v. Sandford, 60 U.S. 393 (1857), the Supreme Court declared that, whether of not someone was a "person" within the meaning of the Constitution was to be determined solely by reference to the Constitution, and Congress and the States had no say in the matter. Specifically, a person descended from a person imported as a slave could never be a "citizen" or a "person" within the meaning of the Constitution.
The decision is widely regarded as one of the worst in the history of the United States. It perpetuated slavery, it lead to the Civil War, and it was directly refuted by the 14th Amendment, one of only four (at most) Supreme Court opinions to have been reversed by constitutional amendments.
The recent Supreme Court decision in Citizens United v. Federal Election Commission, 558 U.S. ___, No. 08-205 (1/21/2010), goes to the opposite extreme, because it holds that anything that a state declares to be a "person" is a "person" for all purposes of the Constitution, and Congress has no say in the matter.
The Dred Scott decision eviscerated the federal government by declaring that whether a slave was a "person" was determined solely by constitutional law and Congress was powerless.
The Citizens United decision eviscerates the federal government by declaring that whether a corporate is a "person" is determined solely by state law and Congress is powerless.
Let's hope that fewer Americans die this time.