If you want to remain idolized and nuke the NCR, you need to make sure you do it before you receive the invite from the ambassador. Otherwise, you'll drop to wild child forever.
Wild Child reputation with the NCR does give you a dialogue option to talk Orion Mereno into standing down, this is normally a 85 speech check but you can use the reputation check instead in Arcade's Quest.
i just need to start a conversation with the ambassador. i havent accomplished the quest "Kings Gambit" and i thought i need to complete it before nuking them