One reason is that the two men have a lot in common. Moreover, neither side wants to derail the chances for greater cooperation at a time of global volatility.
"I have to believe in this kind of high-level meeting, it's not about making anybody feel bad," Ken Hackett, the U.S. ambassador to the Holy See, said in a recent interview in his office here.
Hackett, the longtime head of Catholic Relief Services, has friends in the hierarchy and ties to both parties in Washington. He was seen as a smart pick when Obama nominated him as the new Vatican ambassador last year.
Hackett said there's no doubt that Francis and Obama differ on issues such as abortion rights and gay marriage. But neither he nor Vatican officials expect those wedges to dominate the discussions, nor would either camp want them to sidetrack the meeting.
"I think the president will say that we have principled differences on many issues," Hackett said. "But let's focus on where we find convergences, areas of agreement."
He added: "I don't believe the pope and president will be arguing about why the church doesn't change its position on abortion, because it ain't going to happen. So it's not going to be engaged."
A shared agenda • Instead, the talks are likely to be governed by two other realities. The first is that Francis, the first pope from Latin America, has clearly signaled that he wants the Catholic Church's public witness to focus less on culture-war issues and more on addressing income inequality and helping the poor and society's outcasts which dovetails with Obama's priorities.
"In the past, you had mutual respect and collaboration on important issues" between the Vatican and Washington, said Joshua DuBois, former head of the White House's faith-based office and author of a recent book about his spiritual advice to the president. "But I think for the first time you have a sense that at a high level the priorities are the same and that's new."
The convergences were already clear after Secretary of State John Kerry met his Vatican counterpart in January. The meeting was substantive and positive, and ran well over the allotted time, sources say.
"This was as comprehensive a conversation as I've had with any secretary of state or foreign minister in the course of my tenure," Kerry said at the time, "and I think, happily, we agreed on an enormous amount of things that we can cooperate on."
Hackett and other U.S. officials said they expect Francis and Obama to build on that start by discussing shared concerns such as poverty and immigration, as well as the fight against human trafficking and on behalf of international religious freedom.
Obama made that last topic the centerpiece of his talk at the National Prayer Breakfast last month, though that irked conservatives who see his support for gay rights and contraception coverage undermining religious freedom at home.
But Hackett said that's comparing apples and oranges.
"The narrow discussion of religious freedom that takes place in the United States is important, but when you look at it from the worldwide perspective, there are much more important religious-freedom issues that are looming," he said.
Allies overseas • A second factor shaping the upcoming Vatican meeting is the sudden eruption of violence in global hot spots.
Obama has been drawn somewhat reluctantly into the foreign policy arena while he would prefer to focus on domestic issues. Francis, meanwhile, has sought to push the Vatican back onto the world stage as a voice for peace and a diplomatic player that can play a concrete role as well as providing moral and spiritual guidance.
The ambassador noted in particular the Vatican's role in the Syrian peace talks in Switzerland (and opposing Obama's threatened military strikes), following on Francis' persistent calls for peace in places like Ukraine, South Sudan and the Central African Republic.
What will also be important in the meeting between Francis and Obama is whether they establish the kind of personal rapport that can often shape diplomatic relations and world events. On that score, many expect the pope and the president could hit it off when they spend the first half-hour or so in a one-on-one meeting before bringing in aides to tackle the rest of the agenda.
Vatican officials often speak highly of Obama in private conversations, and even when they disagree with him, they do so in terms that are far less caustic and charged than the language many U.S. bishops use.
For his part, Obama has gone out of his way to praise and quote the pope. That's no surprise: Obama got his start as a community organizer working with the Archdiocese of Chicago fresh out of college an experience that helped shape his views and politics much as Francis' ministry as a priest and bishop in poor areas in Argentina molded him.
"Now there's the foundational element for the president, and here he bumps into Pope Francis, and that's who he is, too," said Hackett. "So I think they're going to have a chemistry that will connect."