Before the party conventions, a survey by the Pew Research Center showed that white Catholics were split on their presidential choice. A similar Pew poll released last week showed that McCain has now opened up a 16-point lead among white Catholics.
A half-century ago, Catholics were considered a reliable Democratic bloc, with seven of 10 Catholics backing John F. Kennedy, making him the first Catholic president.
"There were no theological debates in '60 over birth control, abortion, sexuality," said William D'Antonio, a sociologist at Catholic University in Washington. "There was no ideological division."
But socially conservative voters increasingly felt more comfortable with the Republican Party as those debates raged. How they vote this time could depend on the prominence of the abortion issue.
Biden has recently come under attack from the Knights of Columbus and other groups for his views on abortion rights. Though Biden has opposed public funding of abortion and supported a ban on late-term abortions, he does not advocate overturning Roe v. Wade and has voted often in support of a woman's right to choose abortion.
In a Sept. 7 appearance on NBC's Meet the Press, Tom Brokaw asked Biden, "When does life begin?" In his response, Biden noted there was some debate in the church and referenced St. Thomas Aquinas' Summa Theologia, which quickly drew a harsh rebuke from bishops.
Last month, the Knights of Columbus ran a full-page open letter in USA Today. Signed by Supreme Knight Chris A. Anderson, it offered Biden a choice.
"[Y]ou can listen to your conscience and work to secure the rights of the unborn to share in the fruits of our hard-won liberty, or you can choose to turn your back on them," the letter said.
The advertisement and television appearance ignited the blogosphere, and put other media on notice that abortion wouldn't lie dormant as a campaign issue.
Whelan, the head of the Catholic Democrat group, called the Knights of Columbus letter a "smear" against Biden that fails to "recognize the common ground that exists" on the abortion debate for many voters.
But for others, there is no middle position. Four years ago, Kerry was the subject of intrigue when a handful of bishops condemned his abortion stance and threatened to withhold Communion from Kerry and other pro-abortion rights politicians. Biden faces similar questions - Charles Chaput, archbishop of Denver, urged Biden during the Democratic National Convention to avoid presenting himself for Communion - though the spotlight doesn't shine as brightly on the second on the ticket.
