The Congressional Reasearch Service signals what some already knew. The GOP's Defund ACORN Act could be seen a Bill of Attainder, which is prohibited by something called the Constitution.
A bill of attainder — which is prohibited in Article 1 of the Constitution — is a law targeted to hurt or help an individual. If a bill is regarded primarily as punitive, instead of being strictly regulatory, it could be interpreted as an attainder bill, according to legal experts.
“While the Supreme Court has noted that the courts will generally defer to Congress as to the regulatory purpose of a statute absent clear proof of punitive intent, there appear to be potential issues raised with attempting to find a rational nonpunitive regulatory purpose for this legislation,” CRS legislative attorney Kenneth R. Thomas wrote in an executive summary of the 15-page analysis.
“Thus, it appears that a court may have a sufficient basis to overcome the presumption of constitutionality and find that it violates the prohibition against bills of attainder.”