By Andrew Beatty, and Jean-Louis de la Vaissiere
President Barack Obama and Pope Francis have married their political and spiritual power to urge action on immigration and the environment, during the popular pontiff's maiden White House visit.
America's first black president offered the first Latin American pope a stately and effusive welcome on the South Lawn, praising his moral leadership on issues that politics have struggled to address.
"I believe the excitement around your visit must be attributed not only to your role as pope, but to your unique qualities as a person," Obama said, praising Francis's humility, "embrace of simplicity" and generosity of spirit.
Though Pope Francis has inveighed against the materialism that the United States seems to embody like no other country, he is also a potential political ally for Obama, sharing many of his progressive goals and bringing along many of America's 70 million Catholics.
Speaking in fluent, if accented, English, the 78-year-old Argentine pontiff returned the warm blessings of his host and addressed Washington's bitter debate about immigration reform.
"As the son of an immigrant family, I am happy to be a guest in this country, which was largely built by such families."
Francis said he would address Congress "to offer words of encouragement to those called to guide the nation's political future in fidelity to its founding principles."
Obama lauded Francis for reminding the world that "the Lord's most powerful message is mercy."
"That means welcoming the stranger with empathy and a truly open heart, from the refugee who flees war-torn lands to the immigrant who leaves home in search of a better life," Obama said.
That joint message may also resonate strongly in Europe, which has been convulsed for months by an ongoing refugee crisis.
And as many US conservatives question the very existence of man-made climate change, Francis and Obama made a de facto joint appeal for action on the issue.
"Holy Father, you remind us that we have a sacred obligation to protect our planet, God's magnificent gift to us," Obama said.
Francis took up the call.
"Accepting the urgency, it seems clear to me also that climate change is a problem which can no longer be left to a future generation," Francis said.
"When it comes to the care of our 'common home,' we are living at a critical moment of history."
The pope was afforded a full ceremonial welcome and a 40 minute one-on-one meeting with Obama in the Oval Office.
The visit was a political mirror of Pope Benedict's 2008 visit to George W. Bush's White House. Those two leaders were as conservative as their current successors are progressive.
But Francis has signalled he is unlikely to wade too deeply into America's bitterly fought politics.
The Vatican played a crucial role in brokering talks between Havana and Washington that led to the recent restoration of diplomatic ties after more than half a century.
But the pope told reporters that he would not specifically bring up Washington's embargo of Cuba in his speech on Thursday before American lawmakers, who largely favour taking a tough line with Havana.