If the Holy Father accepts their invitation and appears at the event, as is expected, up to 2 million people could come to Philadelphia, giving the local economy a $100 million shot in the arm, according to early estimates.
"Pope Francis is of course a world leader, and that brings with it a series of steps and events and activities," Nutter said about the Vatican trip. "We have to better understand what the World Meeting of Families is looking for and expecting in Philadelphia."
The delegation has a full schedule of formal events and planning sessions.
On Monday, they will attend a reception at U.S. Ambassador Kenneth Hackett's residence in the Vatican. They'll spend Tuesday with the Pontifical Council for the Family and formally announce that the international conference is coming to Philly. On Wednesday, the delegation will have a "private audience" with Francis.
Nutter said yesterday that he hasn't decided what he'll say to the pope.
"I have not yet figured that out. I plan to have a conversation with my mother, and I'm going to take some advice from my mom about what to say to Pope Francis," Nutter said. Asked if he'll ask for a special blessing for the Phillies, he said, "If I get the opportunity, I'll just ask the pope to bless the entire city, and all of our citizens and our many sports teams."
The group leaves Rome on Thursday, which happens to be the day that President Obama will reportedly come to the Vatican to meet Francis for the first time.
It's unlikely the pope will announce that he is coming next year while the delegation is in Rome. Papal trips are typically confirmed a few months or sometimes weeks before they occur. Also, protocol holds that Obama, as head of state, needs to formally invite Francis before he accepts.
Chaput has said he hopes Francis will give an outdoor Mass, as Pope John Paul II did before more than a million people on the Ben Franklin Parkway in 1979.
The 2015 trip would be the pope's first trip to the United States and is expected to include other high-profile stops, like an address to Congress or to the United Nations.
The background of Philadelphia, where one of the most high-profile clergy sex-abuse cases occurred, and an event centered on family issues could provide a powerful platform for the pope to address those issues.
"The situation that erupted here following the 2011 grand jury genuinely shocked the Vatican because Philadelphia had long been seen as a bastion of tradition and fidelity to Rome, and the place imploded," said Rocco Palmo, a Philadelphia-based writer whose blog, Whispers in the Loggia, is read by Vatican followers around the world. The 2015 event, he said, "will be part of the effort to bring the diocese back."
The World Meeting of Families, which John Paul II began in 1994, will be a six-day conference with events across the city aimed at teaching and discussing the role of families in the church.
The pope has attended every meeting except the 2009 event in Mexico City, which Benedict XVI skipped. At the 2012 meeting in Milan, Benedict first announced that Philadelphia would be next.
After he retired, it was unclear that Francis would follow through on his predecessor's plan. But local officials have said they are confident the pope will decide to come and are treating the Vatican trip as protocol.
The 2015 event is being planned by a nonprofit led by local business leaders, who have the considerable task of raising the enormous sums necessary to host the pope and will travel to Rome with the delegation. The group hasn't yet specified a fundraising target.
Nutter, who was raised Catholic and attended St. Joseph's Prep, a Jesuit high school, will be joined in Rome by his chief of staff, Everett Gillison. Pennsylvania first lady Susan Corbett will accompany her husband, a Catholic who was baptized in Philadelphia.
The delegation will stay in the Grand Hotel De La Minerve, next to the Pantheon. Its expenses will be paid by the 2015 World Meeting of Families nonprofit.
On Twitter: @SeanWalshDN