I had always though Nagin only switched parties because of the truth that New Orleans was 70% black (meaning, 70% Democratic, at base)
One thing people forget about New Orleans is that, in Orleans Parish itself, Bush won the white vote. While it is true they have a gay community, and it is true they are run by Democrats, I never thought of New Orleans as a liberal city. With a few minor exceptions, New Orleans has always been a certain way, few places in America resist change more than it. One thing that could be done is to bring back the New Orleans of my fathers time, the New Orleans that was the one area of Louisiana that actually stood up to Huey Long, etc.
I met Nagin several times, all on business trips, (before Katrina), he seemed to me to be an intelligent man, a man who gave a damn about the city, and he has impressed me in office, most people not from the coast don't realize that more than any Mayor in recent history, he tried to root out corruption, and he cracked down on the brake-tags (I never understood what one of those was)
My relatives down there echo your comments that Nagin was on track to root out corruption and had a somewhat favorable view of him prior to Katrina and he opened up his mouth and lost it on National TV. Regardless of whatever positive things he had previously he will now be regarded as an idiot who couldn't lead under pressure. His legacy is cast. He is trying to undo the damage though. On CSPAN even when he tries to behave himself he still comes across as an arrogant fool. But he does one thing few politicians do - he says what's on his mind and doesn't hold back. Unfortunately, in his case, that's hasn't made him look very good.