The basic argument involves "incorporation". If the US Supreme Court rules in favor of McDonald it would restore the full meaning of the Privileges and Immunities clause in the 14th Amendment of the US Constitution. The clause states, " No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the US". It is important because this clause could then be used, with one broad stroke, to incorporate the entirety of the first eight amendments of the Bill of Rights (aka fundamental rights). As the law currently stands, cases on individual rights are decided upon on a case by case basis in the Supreme Court. Selective incorporation would become obsolete. There would be no more "qualifying" fundamental rights on a case by case basis.
