During one recent television interview, Fiorina was asked about the personal experiences that led her to oppose abortion. She acknowledged that people can have different views, then quickly steered the discussion back to the unhappy state of the economy.
"The central issue in this campaign is the issue that's on voters' minds: What about their jobs?" Fiorina said.
But Boxer is intent on highlighting the distinction between Fiorina and herself on the right of choice, even as the state struggles with an unemployment rate above 12 percent.
Despite a Democratic voter registration edge of nearly 14 percentage points over Republicans, a Public Policy Institute of California poll released Wednesday showed Boxer with just 39 percent support among likely voters compared to 34 percent for Fiorina. An earlier Field Poll also showed Boxer struggling with her lowest popularity rating since she was elected to the Senate in 1992.
Abortion rights, she knows, can help drive voters her way.
"It's a winning issue for Boxer, and a losing issue for Fiorina," said Thad Kousser, an associate professor of political science at the University of California, San Diego.
This is the first time Boxer is being challenged for re-election by a woman. Fiorina, though, is a conservative candidate whose anti-abortion stance runs counter to that of seven of 10 California voters, according to a recent Field Poll.
As in past elections, Boxer is trying to appeal to Republicans and independents who are socially moderate, said her campaign manager, Rose Kapolczynski, including those who supported Fiorina's main challenger in the GOP primary, former congressman Tom Campbell, who is pro-choice.
Fiorina campaign spokeswoman Julie Soderlund noted that the Field Poll found 53 percent of Fiorina's supporters are pro-choice. Soderlund said the number underscores the issue's importance to voters, but that it's not the only issue driving their decision.
Field Poll Director Mark DiCamillo said abortion will work to Boxer's advantage if she can frame Fiorina as a threat to the status quo.
Boxer's political reputation has long been staked on protecting a woman's right to choose in early stages of pregnancy. She supports a ban on late-term abortion as long as there is an exemption when a woman's life and health is in danger.
Boxer, 69, has been endorsed by Planned Parenthood, NARAL Pro-Choice California, Emily's List and the National Organization for Women. A Field Poll released in early July found that women prefer Boxer over Fiorina, 51 percent to 40 percent, while men favor Fiorina 49 percent to 42 percent.
Among independent female voters, Boxer led Fiorina 49 percent to 35 percent.
Fiorina, the former chief executive of Hewlett-Packard Co., believes abortion should be permitted only in cases of incest, rape and danger to a mother's life. The 55-year-old first-time candidate has been endorsed by National Right to Life, California ProLife Council and the Susan B. Anthony List.
Former vice presidential candidate Sarah Palin cited Fiorina's stance on abortion when she endorsed her last spring.
Fiorina has said she derived her view from personal experiences, specifically her inability to have children and her husband's own life story.
"I believe in the sanctity of life. I believe life begins at conception," she said in a television interview. "In my particular case, my mother-in-law was told to abort her child, who became my husband. She chose something different, obviously, and that made all the difference in her life and mine and certainly his."
Kathy Kneer, president of Planned Parenthood Affiliates of California, said most Californians recognize that women should be able to make their own decisions. She said Fiorina's views do not take into account unintended pregnancies, such as those from abusive relationships.
Kneer said Planned Parenthood hopes to raise and spend about $1 million to help get Boxer re-elected to a fourth term.
Fiorina's campaign suggests that Boxer falls far left of the electorate, having voted five times against banning partial-birth abortion. Boxer said she opposed those bills because they lacked an exception for saving the life of the mother or protecting her health.
Emily Buchanan, executive director of the anti-abortion Susan B. Anthony List, said her group will remind California voters about the next senator's role in deciding whether taxpayers should fund abortion. The group contributed $200,000 to Fiorina during the primary and expects to donate another $1 million in the general-election campaign.
Buchanan said the group will target Republican anti-abortion voters, including conservative Hispanic voters, with direct mail and phone calls.
"Taxpayer funding of abortion is what was discussed in the health care bill ... and that message is what resonates," Buchanan said. "Nobody wants to be paying for abortion, no taxpayers."
Fiorina has said she wants to repeal the national health care reforms that President Barack Obama signed into law this year.
Boxer's campaign manager called that a scare tactic. She said taxpayer funding of abortion was limited to cases of rape and incest, or where the life of the mother is at stake, and Boxer is not attempting to change that.
Instead, Kapolczynski suggested Fiorina is a threat to overturning the 1973 Roe v. Wade decision giving women a constitutional right to abortion.
"Are we going to have a senator who defends a woman's right to choose when it's under attack by the right wing," she said, "or a senator who will join forces with the most extreme anti-choice groups to try to make abortion a crime?"