An anti-abortion group in Ohio failed Tuesday in its attempt to gather enough signatures 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 Ohio anti-abortion group collected only about 30,000 of the roughly 385,000 signatures required for a November vote, 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. It seeks to define human life as beginning with fertilization and is intended to ban virtually all abortions, even in cases of rape and incest.

Supporters also fell short of the required number of signatures to qualify for the November ballots in Nevada and California. Voters have rejected similar proposals that made ballots in 2008 and 2010 in Colorado. They also defeated the initiative last year in Mississippi, which has some of the nation's toughest abortion regulations.

Organizers say personhood amendments have a good chance to qualify for ballots this year in Montana and again in Colorado. Each state has a lower threshold of required signatures than Ohio. About 86,000 signatures are needed by early August in Colorado, while fewer than 49,000 are required by Friday in Montana.

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.

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.