According a recent ATV/Zogby poll, Obama has lost the 11-point lead among Catholics that he had in July. That poll showed that Sen. John McCain (R-Ariz.) now enjoys a 15-point lead from Catholic respondents. Other recent polls show a tighter race while the Obama campaign in its daily talking points e-mail points to a June 26 Gallup poll showing Obama winning the Catholic vote 47-43.

Biden has voted in favor of a ban on partial birth abortions, which could trigger concern from abortion rights groups. NARAL Pro-Choice America, which has endorsed Obama, gave Obama a 100 percent rating over the last several years. During the 110th Congress, Biden has rated a 75 with the abortion rights group.

The Catholic Church has strongly condemned abortion and has been critical of lawmakers who support abortion rights.

Losing the Catholic vote frustrated Democrats during the last presidential cycle. In their 2006 book titled "Take It Back," Democratic operatives James Carville and Paul Begala wrote, "You would think the first Catholic presidential nominee since JFK would dominate the Catholic vote as [John F. Kennedy] did. You would be wrong."

Sen. John Kerry (D-Mass.), who is Catholic, lost the Catholic vote in 2004 to President Bush, 52-47. In 1992, the Clinton-Gore ticket won the Catholic vote by nine points and secured a majority of that demographic four years later.

The Gore-Lieberman ticket bested Bush-Cheney among Catholics, 50-47.