Be a supportive partner
If your partner smokes and you’d love for them to quit, have that conversation with them. But bear in mind how you approach it. If you come on too strong and turn it into a lecture, you’re unlikely to get through. Starting the conversation off with pictures of lungs damaged by smoking won’t get you very far either. Your opening gambit needs to come from a place of love and concern. Give your partner space to digest what you’ve said and thought it over. Remember that if they aren’t going to quit willingly, it’s unlikely to succeed. Ask them if they want to discuss it with you more.
Always put it out there that you’re doing this out of concern and care for them. Keep the lines of communication open, and allow your partner to mull things over. It’s possible your partner may take what you say on board and make the decision to quit for themselves. Once your partner has quit, be prepared for the crabbiness that follows during withdrawal. Be patient and let them show you what they need from you. Some people want their partners around all the time; others retreat into themselves for a bit. Make it clear you’re there to support them in whatever way you can.