as a former smoker, and without reading the responses, i feel so bad for you hon. here's what i am thinking ok? first off it is HARD to quit smoking. seriously! it's like, i don't even know what. if someone told you, you could never ever again do the one thing in the world that satisfies you, comforts you, eases your stress, calms you down...and on top of that, something that is completely physically addicting. going through hard times? nope, no comfort for you...don't talk to friends, don't get a hug from someone you love, no way to feel better. stress triggers the need to smoke. i can't even tell you what it's like if you'v never been addicted to something.
that said, i'm not going to say it's "not his fault" and "he's addicted". he is a grown man. he has a choice. if you honestly feel you cannot stay with a man who smokes, that's your choice. but would you leave him to punish him? i'm not sure that's fair. would you leave to make it "easier" on yourself "when" (and IF) he gets cancer and dies a horrible death? he's the father of your child. part of you will always love him, and not being married to him, should this happen, isn't going to make you feel better about it. just think about the reasons behind your threatening to leave him.
i would suggest a bit more tolerant approach. first insist that he doesn't do it in YOUR house, in YOUR car, in YOUR presense. although it sounds like he mostly keeps it away from you. then, realize, he's a grown man. threats and you nagging are never going to make him stop smoking. he would have to decide, FOR himself, BY himself, to quit, for HIS reasons. if you nag him, one of two things will happen. #1, he will quit, because of pressure from you, and then he will start up again and try to hide it. #2, he will just keep right on smoking and it will cause huge upheavals because you will never let it go and your marriage will be screwed. i promise, your nagging won't make him stop. i cannot stress that enough.
NOW, if he WANTS to quit, truly and truly - and it's not just to appease you - then you can be SUPPORTIVE and help him. on HIS terms. ask what HE needs. that's if you really and truly want to help him quit, for real, and for good.
but i'm warning you, until it is HIM that is committed to him quitting, not you, it won't happen. no amount of threats or leaving him will "force" him to quit. if you leave he'll freak out and smoke even more. promise. if you nag, he'll stress out and smoke even more. promise.
he's a grown man. in my opinion, as long as he keeps it away from you, he should be allowed to destroy his body if he wants. it's a free country. but i also didn't vote for all the laws being passed saying no one can smoke in sight of any other human being ever at all in the history of the rest of the world, either (that's what they're doing in my neck of the woods.) this is america. as long as it's not hurting anyone else (phsyically, i mean), i don't feel we have a right to stop someone from hurting him or herself. just mho. i hope you can find peace with this one way or another.