Only person capable of doing a coup is Brown and if there was situation where a coup looked likely he'd go to Blair and ask for resignation before launching it. Even in Blair's weakest moments coup never looked likely see Watson 2006. Blair had the party machine (only Brown had similar power) and the opposition for the most part were useless . So therefore factor in the coup wouldn't be strong opposition but rather external factor and internal dissent so failed Iraq vote maybe but it's unlikely . Blair falling in his sword is more likely