Over the past several decades most of the other provisions of the Bill of Rights, which initially restricted only Congress, have been "incorporated" against the States, meaning their restrictions and guarantees direct the actions of State and local governments as well.
The Due Process Clause of the Fourteenth Amendment was originally intended to make the Bill of Rights applicable to the States, or so the argument goes. However, the Court has not incorporated the Bill of Rights wholesale, but rather one provision at a time, case by case.
McDonald v. City of Chicago is unique among similar enterprises in that the Court may decide to incorporate the Second Amendment not through the Fourteenth's Due Process Clause but rather its Privileges or Immunities Clause, an unprecedented move.
Justice Clarence Thomas, for one, might be more inclined to utilize the latter means rather than the former. Justice Thomas, in addition to being dead set against the incorporation of the First Amendment's Establishment of Religion Clause, occasionally favors reviving legal doctrines that were abandoned 150 years ago (such as that permitting the enthusiastic application of corporal punishment to public school students. See, e.g., "BONG HiTS 4 JESUS").
Last November a group of federal legislators, including Senator Russ Feingold, filed an amicus brief in support of incorporation. Among the arguments put forth is that restrictions on firearm ownership impede the federal government's authority to raise an army:
If many States and local governments followed [the City of Chicago's] example [in restricting ownership of handguns], Congress' ability to exercise its enumerated martial powers would be undermined. Soldiers with previous firearms experience require less training and make better marksmen. And in time of war, the period for training is reduced and pre-existing familiarity with firearms becomes even more important.(The amici stopped short of requiring national firearms training, something France used to do until 1998 and which is still paying dividends in Olympic biathlon medals.)
Wisconsin's other Senator, Herb Kohl, didn't sign the brief (.pdf; 59 pgs.). Wisconsin's Attorney General, J.B. Van Hollen, joined a separate brief (.pdf; 55 pgs.) submitted on behalf of 38 States and arguing in favor of the same objective.
* Erstwhile and future Wisconsin Supreme Court candidate Judge Randy Koschnick believed the question had already been decided.