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I did not think that the President could invoke martial law. That is a states rights issue I thought. He can declare a natural disaster and that is a money issue, but unless the Governor of a state cedes control, the President has no such power!

As a practical matter, anyone can can declare martial law - if they control troops who will back that declaration up with deadly force. Martial law is about raw power. Martial law is about suspending constitutional rights and threatening summary execution of violations of public order. Shoot first and ask questions later, if ever.
In hindsight, we now know that Bush doing that would have been a humanitarian act if it enabled him to accomplish an essentially complete evacuation of NOLA. coconutt2000 makes the point, though, that had he done so, we would be denied the hindsight that we now have - the good he did would be taken for granted, and he would have been impeached for violating the Constitution. Bush himself could not really know, though he certainly must have strongly suspected, that things would have gone this bad when he was between the rock of onrushing Katrina and the hard place of the constitutional limits on his authority.
There was always the bare outside chance that Katrina might actually do limited damage, making the evacuation seem unnecessary in hindsight. That is precisely what had happened on prior near-miss hurricanes, as a caller to Rush Limbaugh noted yesterday. The caller predicted that when NOLA gets dried out we will find many automobiles owned by people whose prior experience of getting out of Dodge in horrendous traffice had convinced them that evacuating wasn't worth it - nothing ever really happened. And that undoubtedly was on the mind of the governor and the mayor as well.
On reflection the best thing Bush could have done would have been to tell the mayor that if he would drive a school bus through NOLA to pick up evacuees, Bush would ride the back seat. Leadership. I don't see how else it could have been done.