(Reuters) - The next major challenge to the constitutional right to abortion in the United States could be a strategically-worded anti-abortion bill gaining momentum in the Oklahoma legislature.

Oklahoma Senate Bill 1433, or the Personhood Act, grants embryos full rights as people from the moment of fertilization. It cleared the Oklahoma Senate in February and is expected to pass in the GOP-controlled House in the coming weeks. The state's Republican governor, Mary Fallin, is an abortion opponent, though she has declined to state a position on the measure.

The legislation is one of a handful of similar initiatives around the country seeking to establish legal rights for embryos, including last fall's failed attempt in Mississippi to enact a personhood amendment to the state constitution.

Like its sister personhood measures, SB 1433 has been controversial within the anti-abortion camp. The initiatives are designed to provoke legal challenges from abortion-rights supporters, with the ultimate goal of giving the U.S. Supreme Court a vehicle to overturn Roe v. Wade, the landmark 1973 decision legalizing abortion. The personhood approach has the backing of such abortion opponents as Republican presidential candidates Rick Santorum and Newt Gingrich, but has been criticized by some anti-abortion leaders, who fear the strategy could backfire.

The measure also poses a quandary for the abortion-rights side. If it chooses not to challenge personhood initiatives, even the symbolic ones, that plays into the anti-abortion strategy of chipping away at Roe. However, if abortion-rights advocates challenge such laws, they run the risk of handing the Supreme Court an opportunity to definitively repudiate Roe.

The wording of the Oklahoma measure appears to have been guided by the more aggressive anti-abortion strategy.

The bill mimics almost verbatim similar legislation enacted in Missouri in 1986, one of the country's first personhood measures. Like the Missouri law, the Oklahoma bill requires the state to give unborn children the same rights and privileges available to other citizens, from the "moment of conception until birth."

But there is a crucial difference. While the Missouri statute explicitly recognized that the rights of unborn children are "subject to the Constitution of the United States, and decisional interpretations thereof by the United States Supreme Court," Oklahoma makes no such acknowledgement. That difference brings the Oklahoma bill into direct conflict with Roe, providing grounds for a lawsuit.

The omission of the "subject to" language certainly looks like it's intentional, said Oklahoma senator Judy Eason McIntyre, a Democrat who voted against the bill in the Senate. The bill's backers seem to have figured, "If it's unconstitutional, it'll be challenged in court," said McIntyre.

Talcott Camp of the American Civil Liberties Union, which supports abortion rights, said the omitted language will be a significant consideration in any decision to challenge the statute. "If the bill becomes law, it's something we would look at very carefully," she said.

The bill's co-sponsors, Senator Brian Crain and Rep. Lisa Billy, have said that the bill simply defines when life begins and, like Missouri's statute, would not prohibit abortion. They did not respond to multiple requests for comment. But activists on both sides of the abortion debate said that by introducing a law that so brazenly conflicts with Roe, Crain and Billy are inviting legal challenges that could ultimately land the case in the Supreme Court.

Personhood USA, the Colorado-based grassroots group behind the failed Mississippi personhood amendment, said the current Supreme Court could uphold a personhood law that restricts abortion, based on the argument that states have greater police powers than the federal government under the 10th Amendment, which grants states powers not delegated to the federal government.

With Oklahoma's Personhood Act, "I think we are on solid footing," said Keith Mason, president of Personhood USA. "The court has given strong indication it's a pro-states' rights court."

The more moderate wing of the anti-abortion movement, however, seeks to chip away at Roe piecemeal, pushing, for example, for state regulations that require parental consent, ultrasounds and 24-hour waiting periods for abortions.

These activists said the aggressive personhood measures are doomed to fail and could ultimately backfire.

Clarke Forsythe, senior counsel for Americans United for Life, a lobbying group, said a personhood challenge would be unlikely to succeed before the current Supreme Court. "We don't have a majority on the Supreme Court right now, and personhood isn't going to change the mind of the justices," he said.

Personhood laws that directly conflict with Roe v. Wade could provoke the Supreme Court to adopt an even more pro-abortion rights posture, striking down restrictions passed by some states, said James Bopp, an Indiana lawyer and general counsel for the National Right to Life Committee, the oldest and largest anti-abortion group in the United States. Bopp labeled efforts like the Oklahoma bill and personhood amendments, "fruitless, imprudent and potentially damaging."

If signed into law, the bill would appear to ban abortion outright and could trigger a number of immediate legal challenges, said Caitlin Borgmann, a professor at CUNY School of Law who specializes in reproductive rights law.

How to best position those challenges is an issue the abortion rights movement is grappling with. On the one hand, they don't want to bring a far-reaching case to the Supreme Court if there's a risk of losing ground on reproductive rights. At the same time, "these measures are so blatantly unconstitutional on their face," said Ryan Kiesel, executive director of the ACLU of Oklahoma.

One strong line of attack against the Oklahoma statute could come from physicians, said Borgmann. They could argue the law is unconstitutionally vague and deters them from engaging in their work -- from performing abortions to treating ectopic pregnancies or administering the morning after pill.

What's uncertain is when or even if a legal challenge could make its way up the courts through the appeals process. When abortion-rights forces sued to block Missouri's personhood law in 1986, the lawsuit, Webster v. Reproductive Health Services, didn't reach the high court until 1989. If the Oklahoma bill passes and is challenged in federal court, it would have to be appealed to the 10th Circuit, and then to the Supreme Court, which would have to agree to hear the case.

Abortion-rights leaders said it was premature to discuss mounting a legal challenge since the bill has not yet become law. But Personhood USA's Mason said that in the past, reproductive rights groups have sued to block personhood amendments at the signature-gathering stage. They will sue to challenge a personhood law "the second it passes," he said.

(Reporting by Terry Baynes)

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