What is the ruling on a smoker leading prayers (Imamah) and delivering sermons (Khatabah), who is struggling to quit smoking, and there is no one else qualified to lead?

Smoking harms the smoker and others. The ugliness of it increases if the smoker is an Imam, due to the smell of smoke that harms the worshippers and the angels. The Prophet (peace and blessings of Allah be upon him) forbade those who ate garlic and onions from praying in the mosque to prevent harm. The smoking Imam must fear Allah and quit smoking. Sincere intention with Allah will help him quit it. Persistence in smoking, despite knowing its prohibition, removes the Imam from justice and makes him a transgressor (fasiq), and it is disliked to pray behind a transgressor. Prayer behind a smoker is valid, but praying behind someone else who is in a better state is more preferable.