WASHINGTON (CNN) -- Of all the critical factors in the November election, pay special attention to Catholics voters. They have an astounding track record, picking the winner in eight of the last nine presidential elections.
There are nearly 70 million Catholics in the United States, about 20 percent of the electorate, and they can tip the balance in a close contest.
They will be listening closely for guidance from Pope Benedict XVI during his first U.S. visit.
"Benedict XVI is not a superdelegate riding into town to deliver a key endorsement," noted John Allen, CNN's senior Vatican analyst. "On the other hand, I think it would also be terribly naive to think there's no political subtext to the Pope's presence in the United States."
Many people wrongly assume Catholics will lean Republican simply because the Pope is so vehemently opposed to abortion rights.
But a recent Pew study found that 51 percent of American Catholics think abortion should be legal in most or all cases. The point is Catholics are not a monolithic voting bloc; they routinely switch between Republican and Democratic candidates for the White House.
"The Catholic vote as a whole is a fascinating study because it is the quintessential swing vote in American elections," said Luis Lugo of Pew. "In 2000 they went, by about 3 percent, for Gore over Bush. In 2004 they went about 7 percent for Bush over Kerry, interestingly who was a Catholic candidate." Watch more on Democrats and value voters »
This is why Democratic Sens. Hillary Clinton and Barack Obama, both in favor of abortion rights, are trying to highlight other subjects where they agree with the Pope.
In CNN's forum on faith last Sunday, Clinton touted that she is in sync with the Holy Father on peace and justice issues.
"He's been a strong voice on behalf of what we must do to deal with poverty," Clinton said. "And deal with injustice." Watch more of Clinton's comments on religion »
Clinton has had an edge among Catholics in the primaries so far, but Obama is trying to shake that up in heavily Catholic Pennsylvania, thanks to the endorsement of Sen. Bob Casey Jr.
Like his late father, the state's former governor, Casey has a major following among Catholics as one of the nation's best-known anti-abortion Catholics.
"Ironically, the first school I went to in Indonesia was a Catholic school," Obama said during CNN's forum. "So, you know, myself and Sen. Bob Casey -- who is sitting here -- we had pretty similar experiences probably." Watch Obama discuss faith and politics »
Republican Sen. John McCain could run into trouble over the Vatican's opposition to the Iraq war, but the Pope has recently expressed concern that a quick U.S. pullout from Iraq could cause a humanitarian crisis.
"I think that McCain's view of the nature and threat of Islamic terrorism is very consistent with what the Pope has said," said Peter Wehner, a former White House aide.
So each side has plusses and minuses as they try to make their case to Catholic voters.
Experts in both parties privately say Catholics could swing to McCain this time -- but only if he stresses social issues like abortion and wears religion on his sleeve a bit more, something he's been uncomfortable doing so far. E-mail to a friend