If the bill passes a narrowly divided Senate in a vote expected today, and is signed by Gov. Michael Rounds, a Republican who opposes abortion, advocates of abortion rights have pledged to immediately challenge it in court - which is precisely what the bill's supporters have in mind.

Optimistic about the new additions to the U.S. Supreme Court, some abortion opponents say they have new hope that a court fight over a ban here could lead to the overturning of Roe v. Wade, the 1973 decision that asserted the legality of abortion around the country.

Not since before 1992, when the Supreme Court reaffirmed a core right to abortion in Planned Parenthood v. Casey, has a state legislature adopted a direct challenge to Roe, said Eve C. Gartner, a senior staff lawyer for Planned Parenthood Federation of America.

Since 2005, bans similar to the bill have been proposed in at least five states, but those on both sides of the abortion debate say the effort here has the strongest chance of succeeding.

The timing of all of this, though, has divided abortion opponents. Some, who argue that a failed Supreme Court challenge now might damage their efforts, have opposed the ban, placing them, somewhat awkwardly, on the same side as abortion rights advocates.

''I don't think it's the most prudent thing to do at this time,'' said Daniel McConchie, vice president for Americans United for Life, a group that opposes abortion rights. ''It's really a long-shot type of situation.''

The proposed legislation, which states that ''life begins at the time of conception,'' would prohibit abortion except in cases where a mother's life was at risk. Felony charges could be placed against doctors, but not against those seeking abortions, the proposed law says.

In South Dakota, advocates on both sides of the ban, which passed the House on a 47-22 vote earlier this month, said they were still uncertain on Tuesday about its fate in the Senate, which has 25 Republicans and 10 Democrats.