For Mamdani, starting off strong is key, said George Arzt, a veteran Democratic political consultant in New York who worked for former Mayor Ed Koch."He's got to use the first 100 days of the administration to show people he can govern," he said. "You've got to set a mindset for people that's like, 'Hey, this guy's serious.'"That push should begin with Mamdani's speech on the day of his inauguration,