A federal judge today struck down Virginia's ban on "partial birth" abortions, saying the law "imposes an undue burden on the right to obtain an abortion."

State Attorney General Mark L. Earley (R), a vigorous opponent of abortion, said he will appeal the decision by U.S. District Judge Robert E. Payne. The case, which entered the federal courts almost as soon as it was signed into law last year, could eventually reach the U.S. Supreme Court.

What opponents call "partial birth" abortion -- a rarely used, late-term procedure -- has been at the center of the national debate over abortion for much of the 1990s, and the ruling on Virginia's law generated reactions that reflect the battle lines drawn in many state legislatures and Congress.

While opponents of the ban acknowledged the judge's ruling would have little if any immediate effect on abortion services, they said they feared the law might have been used to block other abortion procedures. Simon Heller, of the Center for Reproductive Law and Policy of New York City, lead attorney for opponents of the ban, said the law had to be struck down because it was dangerously vague in its scope.

Martin Brown, executive director of the Family Foundation, a Richmond-based group that opposes abortion, said, "Under Judge Payne's logic, he's essentially saying that the third-trimester babies born in Virginia are not viable and can be destroyed.

"This is unconscionable and should not be allowed in a civil society," Brown said.

The law, which took effect last July, banned a third-trimester procedure in which the fetus is pulled out of the birth canal feet first and the skull is partially collapsed so it can be brought through the cervix, the narrowest part of the birth canal.

Payne's decision immediately legalizes the practice, but supporters and opponents agree it won't have a big impact. The procedure is seldom if ever used in Virginia's 25,000 annual abortions, they said, although the state does not keep statistics.

David Nova, president of Planned Parenthood of the Blue Ridge, whose clinic in Roanoke offers abortions one day a week, said the ruling "will have no appreciable effect on the clinic."

Violation of the Virginia law, authored by Del. Roger J. McClure (R-Fairfax), was a misdemeanor, punishable by a maximum of one year in jail and a $2,500 fine.

Payne, who held a trial on the constitutional issue in August, said in his ruling that the law violates the due process clause of the 14th Amendment.

The wording of the law is vague, he said, and "renders it incapable of providing the kind of notice that will enable ordinary people to understand what conduct it prohibits." Also, Payne said, it does not contain an exception for abortions performed after the fetus becomes viable that are necessary to save the life or health of the woman.

Earley said, "We still believe Virginia's statute is well reasoned, legally sound and crafted narrowly enough to pass constitutional muster."

The law, he said, passed the state's General Assembly by a wide margin with bipartisan support "and is overwhelmingly supported by the majority of Virginians."

"Planned Parenthood and certain abortion providers are attempting to use a court of law to accomplish what they could not achieve in the court of public opinion or through the democratic process," Earley said.

Payne's ruling could become an issue in the November legislative elections, when all 140 seats in the General Assembly are on the ballot. The assembly, which reconvenes in January, could attempt to rewrite the law to deal with the judge's objections.

Gov. James S. Gilmore III (R), who signed the bill last year, "stands by the law. It's a horrible procedure," his spokesman, Mark A. Miner, said today. Gilmore also supports Earley's appeal to the 4th U.S. Circuit Court of Appeals, Miner said.

Last year, the 4th Circuit overturned an earlier ruling by Payne in which he granted an injunction that would have temporarily blocked enforcement of the ban.

Virginia is one of 28 states that have attempted to ban the procedure, but in the 20 states where those laws have been challenged, they have been blocked or severely limited by courts, according to the Center for Reproductive Law and Policy.

The center said that Indiana, Kansas, Mississippi, Oklahoma, South Carolina, South Dakota, Tennessee and Utah have laws in effect.

The Maryland legislature defeated a similar abortion bill this year, and none has been proposed in the District.

Congress passed bans on the procedure in 1995 and again in 1997, but President Clinton vetoed both.

CAPTION: Virginia Attorney General Mark L. Earley says the law is "legally sound."