The ruling Monday comes as supporters of the measure are trying to gather signatures to put it before Oklahoma voters on the November ballot. Their effort would amend the Oklahoma Constitution to define a fertilized human egg as a person.
Opponents contend it would ban abortions without exception. They also say it would interfere with a woman's right to use certain forms of contraception and some medical procedures, such as in vitro fertilization.
The state's highest court ruled unanimously that the proposal violates a 1992 U.S. Supreme Court decision and "is clearly unconstitutional."
Dan Skerbitz of Personhood Oklahoma said he had not seen the decision and could not comment on it.
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