by Anthony Kennedy et al.

About the authors: Anthony Kennedy is a United States Supreme Court justice. He wrote the majority opinion in the case Romer v. Evans,which was joined by Justices John Paul Stevens, Sandra Day O’Connor, David Souter, Ruth Bader Ginsburg, and Steven Breyer.

One century ago, the first Justice Harlan admonished this Court that the Constitution “neither knows nor tolerates classes among citizens.” Plessy v. Ferguson, (1896) (dissenting opinion). Unheeded then, those words now are understood to state a commitment to the law’s neutrality where the rights of persons are at stake. The Equal Protection Clause enforces this principle and today requires us to hold invalid a provision of Colorado’s Constitution.

Amendment 2

The enactment challenged in this case is an amendment to...