The President called him and advised him to issue an evacuation order. The governor of Louisiana mentioned publicly the president's call. In the Mayor's own words later he said "we may have a mandatory evacuation order by morning." The director of the National Hurricane Center called him as well pleading with him to issue the order. The Mayor said earlier that he was worried about the legality of issuing a mandatory evacuation order. You can imagine what the President's response would be if the Mayor mentioned legal hurdles during their phone call.
read post #103.
The Emergency Hurricane Plan of the city of New Orleans website specifically states the mayor has the statutory authority to order an evacuation. If there were any legal doubts, don't you think they would have been addressed at the time the plan was being written? He's either lying, or he didn't have any clue what his own Hurricane Plan said.