Barack Obama welcomed Pope Francis to the White House on Wednesday -- the first black US president hailing the first pontiff from the Americas as a unique moral authority.
The South Lawn of the White House echoed to the strains of the Pontifical Anthem, the Star-Spangled Banner and a thundering 21-gun salute, as Washingtonians thronged the streets for the historic visit.
An inspiration to many of America's 70 million Catholics, Francis is also a potential political ally for Obama, sharing many of his progressive goals.
Both men called for action on climate change and hailed the rapprochement between the United States and Cuba -- causes dear to the White House but opposed by US conservatives.
"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 told his guest.
"In your humility, your embrace of simplicity, the gentleness of your words and the generosity of your spirit, we see a living example of Jesus' teachings, a leader whose moral authority comes not just through words but through deeds."
Speaking in fluent but accented English, the 78-year-old Argentine pontiff returned the warm blessings of his host.
"I am deeply grateful for your welcome in the name of all Americans," he said, to applause.
"As the son of an immigrant family, I am happy to be a guest in this country, which was largely built by such families."
In a nod to Washington's bitter debate about immigration reform, 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."
Many US conservatives call into question the very existence of man-made climate change, but 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 on his historic first visit to the United States, and to Washington -- a political city that ordinarily shrugs its shoulders when presidents, queens and sheikhs roll through.
Well-wishers lined the Pope's route and Obama himself made an exceedingly rare ceremonial trip to the airport to meet the Argentine's plane Tuesday, bringing his wife, daughters, Vice President Joe Biden to underscore the special welcome.
The visit is 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.
Still, the White House insists it is not co-opting a holy man in order to batter Republican foes in Congress.
"The goal of this meeting is to give the two men the opportunity to talk about their shared values," said White House spokesman Josh Earnest.
"There'll be time for politics, frankly, the other 364 days of the year," he said.
Francis has signaled he is also unlikely to wade directly 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 also told reporters that he would not specifically bring up Washington's embargo of Cuba in his speech Thursday before American lawmakers, who largely favor taking a tough line with Havana.
"The Holy See is against this embargo, but it is against all embargoes," he said.
Yet there is no mistaking the value of enlisting a popular pope's moral authority and offering him America's largest political platform.
Even the half of Americans who hold an unfavorable view of the Catholic Church like Pope Francis, according to a recent Washington Post-ABC poll.
Francis will make two key speeches during his US visit, the address to Congress and another to the United Nations on Friday.
Topics will include critiques of the dominance of finance and technology; a condemnation of world powers over the conflicts gripping the planet; appeals to protect and welcome immigrants; and climate change, according to Vatican sources.
The pope's agenda tracks so closely with Obama's efforts to introduce immigration reform, as well as domestic and international limits on carbon emissions, that Republicans are already crying foul.
Congressman Paul Gosar, who is Catholic, declared he would boycott the pontiff's historic address to Congress to protest his "leftist" views.
During the historic six-day trip to New York, symbolic home of capitalism, Francis will preside over an inter-faith ceremony at Ground Zero, visit a Harlem Catholic school and greet crowds on a procession through Central Park.
He will wrap up his trip Saturday and Sunday in Philadelphia at an international festival of Catholic families.