Green MP Sue Bradford believes her anti-smacking bill will become law, but by a razor-thin majority.
The bill easily passed its second reading in Parliament last night, by 70 to 51.
And it looks likely to pass its third reading, in about three weeks, when it is expected to received 63 votes.
It needs 61 votes to pass.
The bill would repeal section 59 of the Crimes Act, which gives parents the right to use reasonable force to discipline their children.
Several MPs voted for the bill last night only so they could then support a proposed amendment by National MP Chester Borrows that would define reasonable force.
Six National MPs who voted for Ms Bradford's bill last night are expected to be told by party whips to support Mr Borrows' amendment, although some of those MPs are believed to be strongly opposed to section 59.
Two New Zealand First MPs also supported Ms Bradford's bill so they could then vote for the Borrows amendment.
If those MPs and all those who opposed Ms Bradford's bill last night voted for the Borrows amendment, it would have 59 votes - not enough for it to be passed, but enough to ensure three weeks of lobbying before the bill returns to Parliament.
"I think it will be very close," Ms Bradford said. "It will be very nerve-racking until we can get the bill through the committee stages."
She would fight Mr Borrows' amendment tooth and nail.
"I believe it is the worst possible thing we could do in terms of legitimising the use of force against children. I know Mr Borrows is well-meaning, but unfortunately he, like others who want to somehow define reasonable force, don't seem to accept or understand that this is the worst thing we could do.
"The effect of any attempt to define reasonable force is that we then have the state saying we should hit our kids in some ways and not in others, and that it is still OK to use force on children and babies that we wouldn't consider using on adults."
Mr Borrows said that if the bill was passed, it would make a parent or guardian's smack illegal, no matter how light and justifiable it was.
"I say to those pledging to vote for this legislation right through the process past the third reading, you offer no hope to those you most want to protect," Mr Borrows said.
"If this legislation passes into law without amendment, those it will most affect will be ordinary caring parents who will either smack and feel the guilt of breaking the law, or who won't smack because they don't want to break the law - and it is as plain as the nose on your face that the children of such parents were never in danger."
Parliament's public gallery was crowded for the debate and spectators heard impassioned speeches from MPs on both sides of the debate.
Progressives leader Jim Anderton said the community had once tolerated men hitting women.
Attitudes to that had changed and it was time to change attitudes to hitting children.
Blockhouse Bay single mother Louise Huggins has not smacked her children since 8-year-old Jonathon was 2.
When Jonathon, 6-year-old Robin, and twin 2-year-olds Lucy and Samantha are misbehaving they get a warning. If they ignore that, they go into "time out".
She welcomed the anti-smacking bill passing another parliamentary hurdle last night. "It's probably a good idea if there is a little bit of clarification of what it will mean to parents."
MP Chester Borrows' amendments to alter what constituted "reasonable force" would perhaps allay some parents' fears.
"Everybody is different and we all have our own ways of parenting. But I think there needs to be a line drawn between what is good discipline for a child and what is bad discipline for a child. To be honest, I'm sick of hearing about all the child abuse cases."
Manurewa's Martin Seccombe, a doctor at the Rosehill Christian Medical Centre, and his wife, Samantha, an occupational therapist who home-schools their four children, praise their children for doing good and give them time out for doing bad - and sometimes a smack.
They believe smacking is appropriate for young children such as Elijah, 5, and Georgiana, 4. Baby Talitha turns 1 in three weeks, while talking is usually more effective with 8-year-old Raphael.
Dr Seccombe was disappointed but not surprised by developments last night. "It won't change my view on what's right and what's wrong, and what's best for our children. I think the law as it is provides adequate cover for the base position that assault is wrong. We all agree with that. But parents still need the right to discipline their children."
- Interviews Errol Kiong