The original law, in place for nearly 200 years, was called the Insurrection Act. It allowed the president to deploy regular or Guard troops for police duties whenever laws were not being enforced or the rights of a class of people are being denied because of “insurrection, domestic violence, unlawful combination or conspiracy.”
The 2006 revision describes basically the same state of chaos that might trigger the presidential power. But the circumstances that lead to the unrest are more numerous. They include “natural disaster, epidemic, or other serious public health emergency, terrorist attack or incident or other condition.”
Critics — including the co-chairmen of the Senate National Guard Caucus,
Patrick J. Leahy, D-Vt., and Christopher S. Bond, R-Mo. — said the new law shifted the burden of legal proof onto anyone who would stand in the way of a president grabbing control of the military for domestic police activities.
However, there's some debate about another passage in the new bill about "new procedures for the control of National Guard and active-duty troops during domestic emergencies."