Republican, an abortion rights opponent with a conservative Senate record on the issue, seems content with the public's perception that he's more moderate on the subject.
Democrat, who supports abortion rights, is only too happy to remind voters where McCain stands, but he tries to make his case without attracting too much attention.
Both presidential candidates are gingerly trying to strike the right chord on abortion as they reach out to a critical voting group - independents and moderates, primarily women in swing-voting suburban regions of crucial states such as Pennsylvania, Michigan and Ohio.
The candidates' carefully targeted ads on abortion and stem-cell research, topics that enflame passions among both abortion-rights proponents and opponents, illustrate how Republicans and Democrats alike are tailoring their messages to specific groups of voters.
Obama criticizes McCain in ads that say the GOP nominee takes an "extreme position on choice" and "will make abortion illegal." That misrepresents McCain's position. The Arizona senator favors overturning the Supreme Court's guarantee of abortion rights but would let states decide their own abortion laws, and he is not seeking a constitutional ban.
Obama is using low-profile radio ads and campaign mailings to make his point about McCain. He hopes to avoid being tagged as too liberal on abortion.
McCain, for his part, is responding with radio commercials promising to support stem cell research to "unlock the mystery of cancer, diabetes, heart disease." He doesn't mention that he supports embryonic stem cell research, which many anti-abortion Republicans oppose.
Cultural issues have largely taken a back seat in this campaign to the frail economy and the Iraq war, as well as questions of character and promises of change.
But with the race close, each candidate is trying to woo voters concerned about particular issues in hopes they will help tip the balance on Nov. 4. As with abortion, the candidates are likely to use radio and campaign mail on other social topics, such as gun control and gay rights.
Independent groups also are getting into the act with TV ads. One is trying to make Obama look out of the mainstream even among abortion rights supporters by casting him as far left. Another is encouraging McCain to "embrace a pro-life agenda."
Unlike Democratic presidential nominees Al Gore in 2000 and John Kerry in 2004, Obama has not shied away from using abortion to reach out to independents and moderate Republicans.
Democrats say President Bush's Supreme Court selections of conservatives John Roberts and Samuel Alito gave Obama an opening to press the issue.
"Women are more worried now about the future of the court than in either of the last two election cycles, and Obama has been tapping into that and making this issue a big difference between him and McCain," said Democratic analyst Jenny Backus.
Republicans say Obama doesn't understand that people generally are more conservative than liberal and tend not to be single-issue voters on abortion.
"For the Obama people to try to make McCain into this passionate pro-lifer, it doesn't make any sense," said Rich Galen, a Republican operative.
Obama's radio ad, running in Indiana, Pennsylvania, New Hampshire and elsewhere, features nurse practitioner Valerie Baron telling voters: "John McCain's out of touch with women today. McCain wants to take away our right to choose."
Glossy fliers with the same messages fill the mailboxes of women in Florida, Virginia and other states.
Countering that effort, McCain rolled out his own radio ad suggesting he's not as far right on abortion as Obama makes him seem - though he never mentions that procedure.
Instead, McCain's commercial focused on stem cell research and said he will invest more money in research to prevent disease and find medical breakthroughs to "help free families from the fear and devastation of illness."
Like Obama, McCain backs relaxing federal restrictions on financing of embryonic stem cell research while running mate- along with many conservatives in the GOP - opposes that method because human embryos are destroyed.
McCain opposes abortion rights except when the life of the mother is in danger and, unlike Palin, in cases of rape or incest. He has voted for abortion restrictions permissible under Roe v. Wade and has said the 1973 decision that guaranteed abortion rights should be overturned, leaving states to decide.
Obama and his running matefavor federal money for embryonic stem cell research and largely agree on abortion. But Biden was among a minority of Democrats in voting several times in favor of banning the procedure that opponents call partial birth abortion.