Washington ? Opening debate in Congress, senators hoping to ban a procedure they call partial-birth abortion expressed confidence Monday they have the political clout to prevail after an eight-year struggle.

“I think the odds are very good” of having legislation signed into law, said Senate Majority Leader Bill Frist, R-Tenn., who made the measure one of the top priorities of the new GOP majority.

Opponents conceded as much. And even before the first word was uttered on the Senate floor, the head of an abortion rights organization was looking past President Bush’s promised signature on the bill to a court fight.

“We will challenge it, absolutely, without question,” said Kate Michelman, head of NARAL Pro-Choice America.

Congress has twice passed legislation banning the procedure, in which the fetus is partially delivered before it is aborted, but former President Clinton vetoed the bill both times. Congress appeared ready to pass a third measure in 2000, but halted its efforts after the Supreme Court struck down a state law with many similarities.

The House passed a reworked version of the bill last year, but majority Democrats refused to schedule a debate in the Senate.

Republicans won control of the Senate in last fall’s elections, though, and promptly placed the bill on their list of top 10 priorities. A final vote is expected Thursday.

The White House issued a statement of support to coincide with the beginning of the Senate’s debate, calling enactment “both morally imperative and constitutionally permissible.”

Sen. Rick Santorum, said the legislation aimed to ban a procedure performed only after 20 weeks of pregnancy, and described it in graphic detail. The fetus is partially delivered, he said, and then a scissors is “thrust into the base of the skull and…the cranial contents removed. Just to describe it here has to send shivers down your back,” he added.

He described it as a procedure that is “never medically necessary, not taught in any medical school in this country, not recommended,” yet is performed more than 2,200 times a year.

Sen. Barbara Boxer, D-Calif., quickly countered that Congress should refrain from intruding on a decision between a woman and her doctor.

In addition, she said, the measure was a stalking horse for abortion foes who have a broader goal. “It’s an attempt to outlaw all abortions, to take away a woman’s right to choose…and criminalize abortions,” she said. “And what follows from that? Women and doctors would be in jail.”

The measure that Republicans brought to the floor bans a procedure in which a doctor commits an “overt act” designed to kill a partially delivered fetus.

A Wichita, Kan., doctor, Dr. George Tiller, is one of the few abortion providers who perform the procedure.