Casey, who is a Catholic, has been talking openly about his religious upbringing and his work with the Jesuit Corps. He unveiled a 60-second television ad in the Philadelphia market Wednesday featuring a former student talking about Casey's mentoring from days in the corps' inner-city program.
Similar to a missionary assignment, Casey spent one year with the organization, which he described in a speech last week at Catholic University in Washington.
Religion "is something that a lot of us share, no matter what faith we are," Casey said in a recent interview.
Sharing the spirit
The two-term Santorum, who also is Catholic, has been outspoken on issues consistent with the church's teachings such as opposition to abortion and same-sex marriage. He wrote a book, "It Takes a Family: Conservatism and the Common Good," in which he wrote, "Religious congregations are, by an overwhelming margin, the most important intermediate institution outside the family for the vast majority of Americans."
On Wednesday night in Pittsburgh, close to Santorum's hometown, James Dobson's Focus on the Family Action is sponsoring a "Stand for the Family" rally in support of the religious right. Santorum, who has regular meetings with a religious working group, is slated to address the conservative Family Research Council's voter summit on Saturday.
Religion has been at the forefront of the competitive Pennsylvania race pitting two somewhat similar candidates - both Catholic, both opposed to abortion. It reflects a change from recent elections when many Democrats tended to avoid talking about their faith and essentially conceded religious voters to Republicans. It also highlights a new willingness by Democrats to discuss religion in personal terms.
Other religious revelations
On Monday, Democratic Sen. John Kerry, the party's 2004 presidential nominee, delivered a speech in California in which he talked of "godly tasks" and described his own journey of faith.
In a recent ad in the Tennessee Senate race, Democratic Rep. Harold Ford Jr., seated in a church, tells voters, "I started church the old-fashioned way. I was forced to and I'm better for it."
This spring, Santorum addressed via video a get-out-the-vote training session of the Pennsylvania Pastors Network. Once Casey heard about it, he asked to speak to the group and was given a chance. He did it in person.
"To the extent that he is successful, his campaign might become a model for other Democrats in similar circumstances," said John Green, a senior fellow at the Pew Forum on Religion and Public Life.
Background on Pennsylvania
Casey has held a double-digit lead in polls over Santorum, the Senate's No. 3 Republican, although recent surveys have shown a tightening contest.
More than one-third of voters in Pennsylvania are Catholic. While the state has about a half million more registered Democrats than Republicans, voters tend to be socially conservative and frequently vote along cultural lines.
Santorum has enjoyed political success siphoning the support of socially conservative Democrats - many of whom once voted for Casey's father, the late Gov. Robert P. Casey. The elder Casey, a strong foe of abortion, was revered by many for standing up to the Democratic Party on the issue.
In his son, some Democrats see an opportunity to reach out to voters alienated by the party's stance on social issues.
An August poll by Muhlenberg College Institute of Public Opinion found that among Catholic voters, Casey led Santorum by seven percentage points. Among anti-abortion voters, Santorum led by 17 percentage points.
While both candidates are Catholic and abortion foes, they differ on some issues critical to religious voters. Casey supports civil unions but opposes same-sex marriage. Santorum advocated for a constitutional amendment banning same-sex marriage; Casey said he didn't support it because it was too divisive.
Last spring, Casey addressed evangelicals on global warming at Messiah College in Grantham, Pa. His openness about his faith appealed to at least one voter.
Kimberly Tucker, 48, of Dillsburg, Pa., an evangelical Christian who home-schooled her three children, said she wished other Democrats would follow Casey's lead.
"The Democrats really need to get comfortable with that if they want to appeal to the liberal evangelicals," Tucker said. "They need to be more open with their faith. I think if they are, evangelicals are going to come with them."