LINCOLN, Neb. - It's been called a groundbreaking law, but a measure approved in Nebraska in April that changes the rationale for abortion bans probably won't go into effect any time soon.

Instead, abortion opponents are hoping it will become the most important case on abortion to reach the U.S. Supreme Court in recent memory. Even they acknowledge the ban on abortions at and after 20 weeks of pregnancy - on the assertion that fetuses begin to feel pain at that stage - won't see the light of day in Nebraska unless the high court rules that it is constitutional.

Mary Spaulding Balch, legislative director for National Right to Life, expects that an injunction from lower courts will likely prevent the implementation of the law. The measure, passed by Nebraska's nonpartisan Legislature and signed into law by Republican Gov. Dave Heineman, is scheduled to take effect in October.

Lower courts have no precedent to support the law's basis that fetuses feel pain.

"This is a case of first impression," Spaulding Balch said.

The long trip to the highest court - if it indeed lands there - combined with the time it takes for a ruling there could mean a final decision on the law is several years away.

First, a legal challenge must be posed. No one has stepped forward yet, but Dr. LeRoy Carhart, one of the nation's few late-term abortion providers, is considered a likely candidate.

Carhart, who practices in an Omaha suburb and is the target of the new Nebraska law, was a plaintiff in two of the biggest abortion cases of the last decade that reached the Supreme Court.

Carhart said in a statement that the passage of the Nebraska law and another that requires women to get pre-abortion screenings for mental and physical problems has strengthened his commitment to protecting women's reproductive rights.

The Center for Reproductive Rights, which has close ties to Carhart, hinted in a letter to Heineman urging him to veto the bills that it would be involved in a challenge of the ban on late-term abortions.

"This bill is clearly unconstitutional and is the most extreme abortion law passed in this country in recent memory," the letter states. It reminds Heineman that the center has litigated cases in Nebraska and across the United States in its fight for women's reproductive rights.

The 20-week ban is based on assertions from some doctors that fetuses feel pain by that stage of development.

Critics say there is no firm evidence to support the claim, and that the law is an unconstitutional break from more than 35 years of court precedent that sets viability as the dividing line for abortion restrictions.

Viability is the ability of a fetus to survive outside the womb. While determined on a case-by-case basis, viability is generally considered to be between 22 and 24 weeks of pregnancy.

Opponents of the law also say it is unconstitutional because it doesn't allow mental-health issues to be used as reasons to have abortions at and after the 20-week mark.

Whether an injunction will be issued partially depends on whether a judge believes there is a likelihood that the law won't withstand a legal challenge. Abortion-rights advocates say that standard is clearly met.

The fetal-pain law "blatantly violates" court precedent that abortions can't be banned before viability, so it is likely that an injunction will be issued, said Caitlin Borgmann, an abortion-law expert and professor at the City University of New York.

"What the sponsors and supporters are really hoping for is a test case for Justice [Anthony] Kennedy," she said.

Kennedy, a moderate conservative considered a swing vote on the Supreme Court, is seen by abortion opponents as their best chance for tighter restrictions on the procedure.

While abortion-rights advocates say he has done nothing to suggest he would favor a pre-viability ban, opponents are hopeful because of his positions on two high-profile abortion cases over the past decade. Both involve Carhart.

In 1997, U.S. District Judge Richard Kopf granted an injunction to keep from going into effect a Nebraska-approved ban on the procedure that critics call partial-birth abortion. Kopf later struck down the law, and the U.S. Court of Appeals for the Eighth Circuit upheld his decision. The U.S. Supreme Court, with Kennedy dissenting, upheld that decision.

In 2003, less than an hour after President George W. Bush signed the federal Partial-Birth Abortion Ban Act, Kopf issued an injunction preventing that law from taking effect. As in the late 1990s, Kopf struck down that law and the Eighth Circuit upheld his decision. But the Supreme Court, with Kennedy writing for the majority this time, overturned the decision and upheld the federal ban.