By msnbc.com's Tom Curry

On Tuesday Mississippi voters will decide whether to approve a measure, Initiative 26, that would amend the state constitution to define the word “person” to include every human being “from the moment of fertilization, cloning, or the functional equivalent thereof.”

On the surface, it would seem to be a favorable advance for the cause of abortion opponents but the nature of the measure has sparked concern among some anti-abortion advocates that the passage of the measure could eventually threaten already-existing abortion restrictions.

Mississippi Gov. Haley Barbour, a Republican, told NBC’s Chuck Todd last week that he believes that life begins at conception but “unfortunately, this personhood amendment doesn’t say that. It says that life begins at fertilization or cloning or the functional equivalent thereof.” He said, “That ambiguity is striking a lot of pro-life people here as concerning.”

Nonetheless Barbour later overcame his misgivings and said he voted for the measure when he cast his absentee ballot in advance of Tuesday. He also complained Friday that a group opposing the ballot measure, “has called people's homes and deceived voters into thinking I'm opposed to Initiative 26, the Personhood Amendment. As I've previously stated, I voted for the Personhood Amendment.”

Opinion: Human rights for fertilized eggs? Initiative at odds with science

Despite his vote, Barbour was articulate in explaining why some anti-abortion advocates think the Mississippi measure is either misguided or may lead to unintended consequences. 

He said, “Strategically, there’s some national organizations that think this may mess up trying to get more pro-life policies adopted nationally.”

He also said, “I am concerned about some of the ramifications on in-vitro fertilization (and) ectopic pregnancies, pregnancies outside the uterus in the Fallopian tubes. That concerns me, I have to just say it.”

Jennifer Mason, a spokeswoman for PersonhoodUSA, a Colorado group which is supporting the Mississippi measure, said its proponents “were able to answer his concerns and that’s why he voted for it.”  Mason cited a study by a conservative group, the Mississippi Center for Public Policy, which determined that Initiative 26 would not outlaw in vitro fertilization.

But, in an opinion piece in the Mississippi Business Journal, Jonathan Will, director of the Mississippi College School of Law’s Bioethics and Health Law Center, who opposes the measure, said “If two out of three pre-embryos are lost in the (in vitro fertilization) process, this would seem to be an unacceptable loss of life. If we are committed to pre-embryonic personhood, we should be committed to banning IVF and other similarly risky fertility treatments until such technologies are safe for all persons (including pre-embryos) involved.”

Prominent conservative lawyer James Bopp, who has argued several abortion and free speech cases before the Supreme Court and is the general counsel for the National Right to Life Committee, said that lower federal courts would be likely to strike down the Mississippi measure, if it were enacted, and that the Supreme Court would likely not review the lower court’s ruling.

But if the high court did agree to hear the case, Bopp said, there is a “very substantial danger” that a majority of the justices would adopt a stronger basis for finding that there is a fundamental right to abortion than the due process rationale Justice Harry Blackmun used in the landmark 1973 Roe v. Wade decision.

If that were to happen, Bopp said, the current state and federal restrictions on abortion, such as the Hyde amendment banning federal funding of abortions in the Medicaid program, and laws requiring parental notification before a minor get an abortion, would be swept away.

Bopp sketched out his concerns in a widely circulated memo, pointing to the argument that Justice Ruth Bader Ginsburg made in her dissent in Carhart v Gonzales, the 2007 decision in which the justices upheld the federal law banning the procedure known as partial birth abortion.

A constitutional right to abortion, Ginsburg said, ought to “center on a woman’s autonomy to determine her life’s course, and thus to enjoy equal citizenship stature.”

Mason said Personhood USA’s lawyers think Bopp is wrong. “What we’re expecting to happen with the personhood amendment is that abortion will be made illegal in Mississippi. And that is what the pro-life movement has been working for since the passage of Roe v. Wade -- to ensure that all children in the womb have their personhood rights recognized…. This is a definite way to see some actual results.”

A ballot measure similar to that in Mississippi was rejected by Colorado voters in 2010. Proponents of personhood efforts plan to try to get the measure on the ballot in Florida, Ohio, Oregon and Indiana in future elections.

Updating with a comment from Alexa Kolbi-Molinas, staff attorney with the ACLU Reproductive Freedom Project:

She said the group is hopeful that "voters will reject this attempt to allow government to interfere in the most personal health care decisions of Mississippi’s women and families.  However, should the amendment pass, all options are on the table -- including litigation. We will not stand by while thousands of women and families are placed at risk.”