Available here. Joined by 58 Senators (including 19 Democrats) and 251 U.S. Representatives (including 78 Democrats, although my count here may not be exact). Counsel of Record is former Solicitor General Paul D. Clement. Much of the brief recapitulates the lengthy historical record of congressional action (including but not limited to Reconstruction) to protect the individual right to arms from federal or state infringement.
The brief also points out the incorporation of the Second Amendment is a particularly easy case, in that: 1. Express congressional intent to do so via passage of the 14th Amendment is overwhelming and clear, and 2. Unlike the First Amendment (which begins “Congress shall…”), the Second Amendment does not restrict itself to one level of government, so incorporation should be obvious, given that the First Amendment has already been incorporated.
A third argument involves Congressional powers. First, state gun bans would interfere with Congressional war powers, since research shows that soldiers who have prior civilian familiarity with firearms can be trained faster and to a to a higher level of proficiency. (Sergeant York would be the most spectacular of the millions of examples to prove this point.)
Moreover, Article I already forbids states from disarming the citizens. This is because Article I, section 8, clauses 15-16, give Congress authority over the militia, including the authority to call the militia into federal service. Because of the Supremacy Clause, states may not disable citizens from performing their federal militia duties. Accordingly, the Court accurately explained in Presser v. Illinois, “the States cannot, even laying the [Second Amendment] out of view, prohibit the people from keeping and bearing arms, so as to deprive the United States of their rightful resource for maintaining the public security, and disable the people from performing their duty to the general government.” Thus:
the constitutional design imparts some limits on a State’s ability to restrict the right to keep and bear arms. And it always has.
The Militia Clauses narrow considerably the issue before the Court today. It is unquestionably the case that the right to keep and bear arms limits the States in some fashion. The only remaining question is to what degree. Incorporation against the States of the right to keep and bear arms as reflected in the Second Amendment would have the salutary effect of obviating the development of two different rights to keep and bear arms of different scope. The development of a unified Second Amendment jurisprudence is of particularly practical benefit in light of the relatively nascent state of the jurisprudence due to the confusion sown by United States v. Miller, 307 U.S. 174 (1939). Moreover, incorporation would properly treat the Militia Clauses, the Second Amendment, and the Fourteenth Amendment as a unified whole, codifying the fundamental right to keep and bear arms that the Founders believed pre-dated the Constitution.
The brief goes on to point out the the militia was activated for home defense at recently at World War II, that it could be of use in defense against Mumbai-style attacks, and in any case, the constitutional structure ensures that Congress must always have the choice to use the militia.