Republican Rep. Matthew Hill also defended the legislation on Wednesday.
The bill would ban abortions after 20 weeks if a doctor determines the fetus is viable. Exceptions would exist for mothers who would otherwise die or be severely hurt. Criminal penalties could be included for doctors.
Attorney General Herbert Slatery's opinion says the bill impermissibly subjects doctors to criminal liability while working in good faith to determine what's medically necessary.
It says another constitutional issue is failing to account for severe mental and emotional harm when considering if an abortion is medically necessary.