Well, back in 1878, the Posse Comitatus Act was passed to limit the use of the armed forces in civilian affairs.
It was enacted to withhold the use of the military against factory protests in the North and guarding polls in the South.
It allowed extremely limited use of the military in civilian affairs, except by Constitutional authorization or Act of Congress.
Things like natural disasters or peaceful student protests. Things that civilian law enforcement agencies can't
handle by themselves (they can't tear gas a flood into submission, and they're not allowed automatic weapons to use against
unarmed people).
Then came the War on Drugs. The US Government declared war on the people who are involved in the drug trade.
Forgein groups who smuggle drugs into the country, the citizens who distribute the drugs, the citizens who deal to the users,
and the citizens who are users. By making the declaration of "War on Drugs," the government effectively declared war
on its citizens.
The Coast Guard kept the bad guys out of our waters (as the CIA brought in dufflebags filled with cocaine), while law
enforcement officers (trained by the military) conducted raids on American citizens. (Occasionally raiding the wrong
home.)
Now we have the War on Terrorism.
Things like chemical, biological, and non-conventional warfare are strong possibilities. By looking to expand the
Posse Comitatus (as much as it would pain them), they are making a clear statement that they don't trust the masses.
Law enforcement, even at the federal level, is insufficient and useless against such a large and faceless enemy. Military
advisors were brought in during the Waco seige of the Branch Davidian compound. (Like they were needed.) Law enforcement
has been blurring the lines between public security, peacekeeping, and the use of US troops against US citizens for years.
(Two words: Kent State.)
There have not been any direct uses of Posse Comitatus against civilians (in terms of search and seizure or arrest) that
the Supreme Court could use to test its Constitutionality. Congress could easily declare the War on Terrorism a "state
of national emergency" (even though every warning has been just a warning) and override the other branches of government to
declare Martial Law.
The wheels have been slowly turning in that direction for decades.
(I'm not sure that Ridge meant to publicly say anything about Posse Comitatus... The way that he backed away from
his statement makes me wonder if he didn't let something slip...)