SIOUX FALLS, S.D. — Kayla Brandt had an abortion three years ago and instantly hated having done it. Now, hoping to stop other women from making the same choice, she is a public advocate for the most severe abortion ban in the nation.

“I don’t want anyone to feel what I did,” Ms. Brandt says.

Maria Bell is a Sioux Falls obstetrician-gynecologist who also joined the political fray for the first time, but Ms. Bell is on the opposite side. Appalled by the attempt to shut the state’s only abortion clinic, she says she would not be able to live with herself unless she worked to overturn the law.

“To think passing a law will stop abortion is incredibly naive,” Ms. Bell said.

South Dakota is the unlikely home of this year’s most intense duel over abortion, a November 7 referendum to decide the future of HB 1215, a measure that would institute a broad ban on the procedure. No exceptions would be allowed for pregnancies resulting from rape or incest — abortion would be permitted only when the mother’s life is in jeopardy.

Partisans across the nation are delivering money and tactical advice on an issue that has divided residents of the state. South Dakota’s fight could be a harbinger of political battles across the country should the Supreme Court strike down Roe v. Wade, the 1973 decision that legalized abortion nationwide.

“This has become the focal point in the country for the choice debate,” Nancy Keenan, the president of NARAL Prochoice America, which is channeling cash into the campaign, said. “The stakes are very high, especially for us to win in November and again say America is prochoice, America doesn’t think politicians should be involved in these private decisions, and enough is enough.”

A fresh poll suggests voters are inclined to oppose the law as too severe. In a late-July sounding, opponents of the ban held an 8% lead, with 14% undecided.

“There’s going to be a lot of money spent,” state Rep. Roger Hunt, a Republican and the ban’s chief sponsor, said, urging voters to defend the unborn. Pointing to the vivid statewide conversation over HB 1215, formally known as the Women’s Health and Human Life Protection Act, he said, “There are a lot of people who have gotten very political because of HB 1215.”

People such as Ms. Brandt, a 29-year-old financial auditor, who described a period of quiet misery after an abortion about three years ago. When the doctor finished, Ms. Brandt said, she felt an emptiness that led to a long year of grief.

Later, she decided to speak out, hoping to create what she called a “haven” for women and children in South Dakota by outlawing abortion. Her smiling face now appears on the letterhead of VoteYesForLife.com, the umbrella group mobilizing support for the ban.

“I was in a relationship and panicked and got scared and ashamed and thought an abortion was the means to fix my mistake,” Ms. Brandt, who came to see herself as a “mother who was sadly stripped of her child,” said.

“Where’s the baby’s choice?” Ms. Brandt asked. “What about the life of the baby?”

The bold South Dakota strategy has energized some proponents while highlighting strategic splits in the anti-abortion movement. Many committed foes, focusing on incremental steps to make abortion less accessible, believe that Roe v.Wade cannot realistically be challenged until the composition of the Supreme Court shifts further.

The ban is not “something we would have chosen,” the chief of staff of Chicago-based Americans United for Life, Daniel McConchie, said. “To overturn Roe v.Wade, which is the goal here, you have to be able to count to five members of the court. We count five in favor of keeping Roe.”

Mr. McConchie considers it unlikely that any one case — even a near-total ban on abortions — would force the high court to reconsider the landmark decision before the justices are ready. Nor, he believes, is such a law necessary to get the Supreme Court’s attention.

Performing an abortion in South Dakota would be a felony if the mother’s life is not in danger, according to the law, which declares that mother and fetus “each possess a natural and inalienable right to life.” No exception exists for rape, although rape victims would be permitted to take morning-after contraceptives “prior to the time when a pregnancy could be determined through conventional medical testing.”

Governor Rounds, a Republican, signed the bill into law in March, declaring that the unborn are “the most helpless persons in our society.” Architects of the law never really expected it to be implemented. Instead, they figured that it would be the subject of a lawsuit that would eventually make its way to the Supreme Court, where they hoped it would be upheld by the overturning of Roe v. Wade.

But instead of suing to block the law, opponents are using a 19th-century provision that allows voters to overrule the legislature by referendum. Meanwhile, the law is on hold.

In a socially conservative state of 775,000 residents who twice gave President Bush 60% of the vote, abortion defenders gathered more than 38,000 signatures — more than twice the number necessary — to place the measure on the ballot. Supporters drew on traditional abortion rights advocates, as well as Republicans who feel the legislature is too intrusive.

Also on the November 7 ballot is a constitutional amendment that would define a marriage as being between a man and a woman while prohibiting the recognition of “civil unions, domestic partnerships, or other quasi-marital relationships between two or more persons regardless of sex.”