If asked by the Governor or the legislature the President could enact the "Insurrection Act" and send in the armed forces to keep the peace. If that is not done, then armed forces are prevented from "enforcing state laws" by posse comitatus.
NG on the other hand, are under the command of the Governor of the State and they along with local police and state police can enforce law.
I heard Blanco say today that law officers coming from other states would be deputized in LA, therefore making them able to enforce the law.
An infrequently cited constitutional power of the president provides an even broader basis for the president to use military forces in the context of homeland defense. This is the president’s inherent right and duty to preserve federal functions. In the past this has been recognized to authorize the president to preserve the freedom of navigable waterways and to put down armed insurrection. However, with the expansion of federal authority during this century into many areas formerly reserved to the states (transportation, commerce, education, civil rights) there is likewise an argument that the president’s power to preserve these “federal” functions has expanded as well. The use of federal troops in the South during the 1960s to preserve access to educational institutions for blacks was an exercise of this constitutional presidential authority.
Since the TITLE 18 > PART I > CHAPTER 67 > § 1385 of the US Code is a law passed by Congress, Congress can also choose to adopt a resolution altering it for any specific case.