ACALIFORNIA federal judge's ruling last week overturning the ban on the procedure known to opponents as partial birth abortion is far from the final word on this divisive subject. Challenges to the constitutionality of the new law are making their way through two other federal courts, and it seems inevitable that the Supreme Court will be called on to make the final judgment.

But U.S. District Judge Phyllis J. Hamilton's far-reaching ruling, invalidating the law on three constitutional grounds, suggests why the justices should be no more receptive to the congressional measure than they were to a Nebraska partial birth abortion ban they rejected four years ago. Judge Hamilton's exhaustive assessment of the medical evidence underscores the degree to which the new law, like its Nebraska predecessor, infringes on women's ability to obtain other late-term abortion procedures and could prevent doctors from using the method they deem best to safeguard their patients' health and capacity to bear children.

The partial birth abortion procedure, also known as intact dilation and extraction, is a variant of the hardly less gruesome dilation and evacuation procedure used in the vast majority of abortions after the first trimester of pregnancy. Judge Hamilton found that the law -- though more tightly written than the Nebraska statute -- remained so broadly worded that it could apply to regular D&E abortions or even those performed by inducing labor. Therefore, she said, it imposes an undue burden on a woman's ability to obtain a second-trimester abortion. She also found it unconstitutionally vague.

Perhaps most significant, though, Judge Hamilton rejected Congress's attempt to make an end run around the Supreme Court ruling by declaring that the partial birth procedure is never medically necessary to protect maternal health. The absence of a health exception was one of the high court's chief reasons for invalidating the Nebraska law. Congress sought to sidestep this requirement by asserting its own medical judgment that doctors would never need to resort to the procedure on that basis; indeed, Congress asserted, the procedure "poses serious risks to the long-term health of a woman and in some circumstances, their lives."

Judge Hamilton cited extensive evidence to the contrary. She found that the technique could be safer than other methods in some instances: It can reduce the risks of perforating the uterus or causing complications by leaving fetal parts behind, and it can be performed more quickly, lessening blood loss and the possibility of infection. In some situations -- for example, where there is a fetal anomaly such as hydrocephalus -- the partial birth method might be preferable to alternatives such as a Caesarean delivery, which could compromise the woman's future childbearing.

Congress is entitled to some degree of deference in its assessment of the factual basis for legislation. This law, though, is an example less of diligent fact-finding than of ideological fact-twisting; its backers relied on the opinions of anti-abortion physicians, disregarded conflicting facts and misrepresented others. Judge Hamilton was the first jurist to call them on this. She's not likely to be the last.