Today, the Arkansas legislature did something unprecedented: It overrode a governor's veto to ban all abortions after 12 weeks' gestation.

The ACLU of Arkansas has promised to challenge the restriction in court, setting up a potentially interesting battle over what right to abortion Roe v. Wade actually protects.

In that landmark decision, the Supreme Court recognized viability — the point when a fetus could live outside the womb — as when states retain the right to ban elective abortions.

"If the State is interested in protecting fetal life after viability," the court wrote in Roe, "it may go so far as to proscribe abortion during that period, except when it is necessary to preserve the life or health of the mother."

The Supreme Court reaffirmed this standard in its 1991 ruling, Planned Parenthood of Southeastern Pennsylvania v. Casey, in which it specified viability as "the earliest point at which the State's interest in fetal life is constitutionally adequate to justify a legislative ban on nontherapeutic abortions."

Since 2010, eight states have passed laws that appear to violate this standard. They ban abortion at 20 weeks, when viability is generally thought to occur not until 22 to 24 weeks. Legislators in those states have argued that, at 20 weeks, a fetus can feel pain — and that fetal pain, rather than viability, ought to be the standard by which abortion bans are judged.

A challenge to Arizona's 20-week ban has centered on this question, with supporters of that restriction arguing that fetal pain ought to be the standard for deciding when abortion can, and cannot, be outlawed.

The Arkansas measure does something, however, completely different: It attempts to redefine viability altogether. "Viability," Senate Bill 134 states, "means a medical condition that begins with a detectable fetal heartbeat."

In a weird way, the Arkansas law actually doesn't really try to challenge the viability standard as the 20-week bans do. Instead, it tries to rewrite what viability actually means.

Will that standard fly in the Supreme Court? It would near certainly require the justices to throw out Roe's central holding, that until "the end of the first trimester, the abortion decision and its effectuation must be left to the medical judgment of the pregnant woman's attending physician."

Still, it's a notably different tactic than other states have taken in their attempts to ban abortion. And it's one that, starting today, will likely wind its way up through the American legal system.