WASHINGTON -- One of the more intriguing dynamics of this year's Pennsylvania Senate race is that when faced with a Democrat and a Republican who both oppose abortion, some Democratic women are going to sit the race out.

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The issue was thrust to the forefront this past week when several women's groups endorsed a virtually unknown Democrat, Alan M. Sandals, as their choice to challenge Sen. Rick Santorum, largely because Mr. Sandals supports abortion rights.

In interviews, members of the National Organization for Women and other women who are staunch supporters of abortion rights, said they were still seething over party leadership's decision to push the candidacy of state Treasurer Bob Casey Jr. Some said they felt abandoned by their party and that they would skip that race on the ballot in November and wouldn't be opening their checkbooks as they have for other Democrats.

For now, Mr. Casey appears to be doing fine without them. He is clobbering Mr. Santorum in the polls, largely because of the perception that he can beat the incumbent Republican senator. His treasury is robust and the party leaders who ushered him into the race are working to soothe the concerns of abortion rights advocates within the Democratic base.

But the question of how many women will stay home is creating interesting calculations in the campaign war rooms. One is whether the desire among Democrats to topple Mr. Santorum can balance out the women the campaign will lose on abortion. Another is whether Mr. Casey's positions on abortion and gun rights can bring back significant numbers of conservative Democrats who have been voting Republican.

In 2004, when Pennsylvania's electoral votes hung in the balance, President Bush and Sen. John F. Kerry raced through narrowly divided areas such as the Philadelphia suburbs, competing for what became the archetypal Pennsylvania swing voter: Republican women who tilted toward Mr. Bush on national security issues, but agreed with Mr. Kerry on abortion rights.

Though Mr. Casey and Mr. Santorum still are likely to compete for that trove of unpredictable voters, abortion won't be the swing issue.

Like Mr. Santorum, Mr. Casey believes Roe v. Wade should be overturned. He doesn't think the federal government should expand its support of embryonic stem cell research beyond the policy Mr. Bush laid out in 2001, and he said he would have voted to confirm Supreme Court Justice Samuel A. Alito Jr., who was opposed by 41 of the Senate's 44 Democratic senators.

But Mr. Casey's campaign manager, Jay Reiff, said he believed many Democratic women who favor abortion rights would side with his candidate because of related issues on which the two candidates disagree. For example, Mr. Casey supports making emergency contraceptives available and wants to boost federal funding for family planning in the United States and abroad, while Mr. Santorum does not.

"[Mr. Casey] believes that there is a consensus in this country from President Bush to Senator Clinton that we should be working toward reducing abortions ... and often that consensus gets overlooked," Mr. Reiff said.

Mr. Reiff predicted that Mr. Casey would do better across the state and in the rural middle of the state than more liberal Democrats have done because of what he sees as the consensus on the issue.

"I think it kind of all balances out," Mr. Reiff said.

But Mr. Santorum's supporters are eager to play up the fissures among Democrats at this early stage.

Sen. Arlen Specter, a Pennsylvania Republican who has benefited mightily from the crossover vote on abortion rights, which he supports, said this week that Mr. Casey's views on abortion would help Mr. Santorum.

"There's a lot of angst in the country and in the state about a woman's right to choose. People are very edgy about what [Chief Justice John] Roberts and [Justice] Alito are going to do," said Mr. Specter, who oversaw the Supreme Court confirmation hearings as Senate Judiciary Committee chairman.

"The people who really care about that issue are going to stay home [this fall]," Mr. Specter said. "Maybe not all of them will stay home, but there won't be enthusiasm, they won't be getting their friends out."

One of those people is Jeanne K.C. Clark, of Shadyside, who heads a group known as Women Leaders Online and says she corresponds constantly with women across Pennsylvania who share her frustration with Mr. Casey's views on abortion, stem cell research, the Patriot Act and Justice Alito.

"He doesn't just need people to vote for him, he needs people to work for him," Ms. Clark said of Mr. Casey, adding that when she goes to the polls this November, she expects to leave the Casey-Santorum choice blank.

"I have been involved in every campaign since I turned 21 ... and I know that, where I live in Pittsburgh and across the state, the Democrats poured out their hearts for John Kerry. And they did it for [Governor] Ed Rendell, but they didn't do it for [2000 Democratic Senate candidate] Ron Klink, and they're not about to do it for Bob Casey."

There are many Democrats who do not agree with that sentiment. Some strategists note that while Mr. Santorum defeated Mr. Klink, who was anti-abortion and pro-gun rights, the incumbent senator led by significant margins in most polls throughout the race.

They note that Mr. Casey's father, who was far more outspoken than his son about his opposition to abortion, swept the 1990 governor's race by a huge margin.

So to allay concerns among some Democrats about the abortion issue and to make sure they stay involved, prominent members of the party such as Mr. Rendell and Democratic U.S. Rep. Allyson Y. Schwartz are talking up what they see as the strengths of Mr. Casey's record on women's issues.

Ms. Schwartz, an abortion rights supporter who represents Montgomery County and Northeast Philadelphia, said women who care about abortion rights "will also look at who can they trust to stand up for quality education, access to health care for themselves and their children, and who can they better trust to care about issues like child care."

"In every one of those categories, Bob Casey comes out ahead," Ms. Schwartz said.

And the decision-making process of women such as Kate Michelman, the former head of the abortion rights group NARAL who considered running in the Pennsylvania Senate race as an independent, might become a template for the women who share her views.

"It was very hard for me, personally, to decide what the right thing to do is, and I think for many voters in Pennsylvania, pro-choice or concerned about the future of our personal and religious liberties, this race has caused some conflict," Ms. Michelman said.

"In the end, defeating Santorum as an important step in taking back the country from the forces of extremism became a principle unto its own," she said.

But asked whether she would work to get Mr. Casey elected, Ms. Michelman said that was one decision she had yet to make.