It started with Lincoln, who declared an emergency and delegated dictatorial powers to himself, and Congress didn't stop him. That was actually when the republic died; since then it's been a charade. I haven't studied it myself, but I've read that every president since Lincoln has renewed the "emergency" he declared, which supposedly is what gives them the power to rule by decree, as Lincoln did. Dubya didn't create this situation, he has only made efficient (and far more open) use of it.
The fact is, there is no provision in the Constitution for any kind of "emergency" or for the president to be granted special powers in any case. There was such a provision in the constitution of the Roman Republic, where the Senate could give one man special, essentially unlimited powers for a limited period of time (six months or so, I think it was) to deal with an emergency situation. Such a man was called a "dictator", and that is where we get this term. This also included being protected from prosecution for anything he did during that time.
Sulla was the first to be given that power, I think, and he used it to slaughter everyone he didn't like. When his term as dictator was over, he simply retired to the country. Julius Caesar was also granted dictatorial powers for a couple of years, but when it began to look like he was going to assume the position of a king -- thus returning Rome to what it was before the Republic had been founded centuries before -- a group of republican conspirators assassinated him. Which led, ironically, to the final collapse of the Republic, civil war, and finally restoration of order under Imperial rule.
Officially, of course, the Republic lived on, with a "functioning" Senate -- which respectfully requested the various emperors to manage the Republic's affairs during the state of emergency -- which lasted for another 400 or so years, until the barbarians finally completely overran Rome.
The Founders, of course, being educated men, were all very familiar with this history -- which is probably why they made no provision for "emergencies" or a "dictator" in the Constitution. Just as "war is the health of the state", the "emergency" is the natural habitat of the political insect.