OKLAHOMA CITY — Citing concerns about vague wording and about a potential legal battle over its constitutionality, Gov. Mary Fallin on Friday vetoed a measure that would have locked up doctors who perform abortions.

“The bill is so ambiguous and so vague that doctors cannot be certain what medical circumstances would be considered ‘necessary to preserve the life of the mother,’” Fallin said in a statement announcing a rare veto on an anti-abortion issue. The measure would have made it a felony for doctors to perform abortions and challenged a 1973 U.S. Supreme Court ruling which first legalized the practice.

Opponents of Senate Bill 1552, which also threatened to revoke the licenses of doctors, said it amounted to an outright ban on abortion. Even before the bill ended up on her desk, Fallin’s spokesman said nearly 2,900 people had contacted her office by early May to express their thoughts on the legislation.

Fallin faced mounting pressure from pro-abortion groups to veto the measure. Organizations— including the American Civil Liberties Union of Oklahoma — said a lawsuit would be imminent if the first-of-its-kind measure had became law.

One of the bill’s authors, Nathan Dahm, R-Broken Arrow, said he had no comment following the veto announcement, but said he was considering whether to try to override Fallin’s veto.

Lawmakers could still overturn it if two-thirds — or 68 in the state House and 32 in Senate — vote in favor of the measure.

Ryan Kiesel, executive director of the ACLU of Oklahoma, said if it ultimately becomes law, the measure is “ripe for a challenge.”

“It flies in the face of nearly four decades of judicial precedent that guarantees a woman’s right to access abortion care,” he said. “I think it’s all but a certainty that this will be challenged in court if it becomes law. I think the only question is who’s leading the lawsuit.”

In a statement, The Center for Reproductive Rights noted that since 2011, Fallin has signed 18 bills “restricting access to reproductive health care.” The group said it challenged what it believed were unconstitutional restrictions on reproductive health care in Oklahoma eight times in five years. State laws that shut down clinics, banned a common method of second-trimester abortions, restricted medication abortions and forced abortion providers to perform ultrasounds and also display and describe the image were all blocked by courts, the group said.

Kiesel said he wasn’t surprised the bill ended up on Fallin’s desk.

“Those measures are getting more and more extreme every year,” he said. “It doesn’t surprise me that politicians are playing politics with the health of Oklahoma’s women. It does surprise me the length they’ll go to play those political games.”

Rep. Lewis Moore, R-Edmond, who co-authored the measure, said vast majority of Oklahomans share his values.

“If you are pro-life, it comes down to ultimately, ‘Are you willing to stop abortions, and how far are you willing to go to do that?’ This takes it to that logical step,” he said, adding the measure would have outlawed all abortions except those deemed medically necessary.

That wording was a concern for the doctors in state Sen. Ron Sharp’s district.

Though he ultimately voted for the measure, the Shawnee Republican removed his name as co-author after physicians reached out. Sharp said he struggled to balance his constituents’ anti-abortion views and doctors’ needs in his district.

“The doctors think it’s a little too radical,” he said. “They understand … how this is an issue that relates to Republicans, but at the same point in time putting a doctor in the penitentiary when he has to make a decision under the vagueness of that language as to when a woman’s life’s in danger, you can’t really put that into legislation.”

Janelle Stecklein covers the Oklahoma Statehouse for CNHI’s newspapers and websites. Reach her at jstecklein@cnhi.com.