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An anti-abortion group fell short Tuesday in its attempt to gather enough signatures to ask voters this fall to change the state constitution to declare that life begins when a human egg is fertilized.

Backers of the proposed constitutional amendment in Ohio, a key state in the upcoming presidential election, and elsewhere hope to spark a legal challenge to the U.S. Supreme Court's landmark 1973 Roe v. Wade decision, which upheld a woman's right to an abortion until the fetus is viable outside the womb, usually at 22 to 24 weeks.

The group had collected only about 30,000 of the roughly 385,000 signatures required to be submitted to state officials on Wednesday for a chance to appear on November ballots, said Patrick Johnston, the director of Personhood Ohio.

The group pledged to continue seeking signatures ahead of 2013, but the shortfall was another setback for what has become known as the "personhood" movement.

"I did not get enough (signatures) in the mail today," Johnston said Tuesday. "It's going to take a little longer."

Supporters fell short of the required number of signatures to qualify for the November ballots in Nevada and California. And in Oklahoma, the state's highest court halted an amendment effort there to grant personhood rights to human embryos, saying the measure was unconstitutional.

Voters have rejected similar proposals that made ballots in 2008 and 2010 in Colorado. They also defeated the initiative last November in Mississippi, which has some of the nation's toughest abortion regulations.

Organizers say a personhood amendment has a good chance to qualify for the fall ballot again in Colorado. The state has a lower threshold of required signatures than Ohio; about 86,000 signatures are needed by early August.

The measures vary in some details, but in general they define human life as beginning with fertilization and are intended to ban virtually all abortions, even in cases of rape and incest. Many physicians have said the measures could make some birth control illegal and deter in vitro fertilization.

Supporters in Ohio had hoped to alleviate those concerns by rephrasing their proposed amendment to say it wouldn't affect "genuine contraception" or in vitro fertilization procedures.

"These kinds of things are going to grow by increment," Johnston said. "We have to change people's minds one at a time."

The proposal hasn't attracted support from some traditional supporters of Ohio's anti-abortion measures.

The anti-abortion group Ohio Right to Life is not involved. And the Catholic Conference of Ohio, while it hasn't taken a formal position, is not encouraging its 785 parishes to gather signatures.

Johnston said reluctance from the two groups to help out with the personhood amendment in Ohio, whose constitution doesn't define life, created a challenge.

"We have a mission that's internal in some ways," he said.

The amendment also is trying to gain traction at the same time as the so-called Heartbeat Bill, which would outlaw abortions at the first detectable fetal heartbeat, sometimes as early as six weeks into pregnancy.

Activists from across the political spectrum say getting an issue on Ohio's ballots is a daunting task.

The We Are Ohio campaign, which successfully sought the 2011 repeal of the state's collective bargaining law, hired paid signature gatherers even though it already had the support of well-funded labor unions, the Ohio Democratic Party and thousands of volunteers.

Johnston said his group isn't drawing money from big donors, nor is it using paid circulators.

Leaders in Personhood Ohio planned to meet next month to discuss how best to move forward and what changes could be made to their strategy.

Ohio is expected to be a pivotal, hotly contested state in the Nov. 6 presidential election. But Johnston said he wasn't sure whether this fall's ballot was the best shot at getting the amendment approved by voters.

"We're really trusting in the Lord," he said. "We don't know what the future holds."

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Associated Press writer Andrew Welsh-Huggins contributed to this report.