After bonding with US President Barack Obama on progressive themes like climate change, Pope Francis on Thursday addresses Congress, where conservatives embrace his moral authority but are less comfortable with the pontiff's more political preachings.

On his first full day in the United States, Francis was greeted with pomp and ceremony at the White House Wednesday, then crisscrossed Washington to huddle with bishops, greet ecstatic well-wishers, conduct mass and elevate a Franciscan friar to sainthood.

He takes America's grandest political platform at 1400 GMT, when he becomes the first leader of the Roman Catholic Church to address a joint meeting of Congress, including more than 500 lawmakers, Supreme Court justices and top administration officials including Vice President Joe Biden.

Following his 40-minute speech, Francis will emerge on a balcony overlooking the Capitol's west front lawn, where 50,000 people will gather.

Republican lawmakers anticipate Francis's stance opposing gay marriage and abortion, while Democrats are elated at the prospect of his backing on progressive issues including immigration and the fight against economic inequality.

- Pope 'transcends' politics -

House Speaker John Boehner expects the pontiff to rise above US political bickering and provide bipartisan inspiration.

"The pope transcends all of this. He appeals to our better angels and brings us back to our daily obligations," Boehner said Wednesday.

"The best thing we can all do is listen, open our hearts to his message and reflect on his example."

Some 30 percent of US lawmakers are Catholic, while nine in 10 are Christian, and the pope is widely admired by Republicans and Democrats alike.

Buildup to the speech has not been without controversy, however. Several Republicans have expressed wariness about Francis bringing a politicized message. One congressman said he will boycott the pope's address.

"In theological matters, as a Catholic, I believe that... he's infallible in those decisions," Senator Marco Rubio, a 2016 Republican presidential candidate, told Fox News.

"On moral issues he speaks with incredible authority, and he has done so consistently, on the value of life, on the sanctity of life, on the importance of marriage, and on the family," he added.

"On economic issues, the pope is a person" just like any other.

Senator John Thune said he hoped lawmakers could learn from the pope about mercy and compassion.

But he added that "there are also some things we hope that he can take away from visiting our country for the first time."

Among them, Thune said, was "recognizing that free enterprise is the greatest anti-poverty force that the world has ever seen."

Francis said on his flight from Cuba to Washington that he likely would not bring up the US embargo against the island nation, which Republicans in Congress oppose lifting.

The Vatican played a crucial role in brokering talks between Cold War foes Havana and Washington that led to the recent restoration of diplomatic ties.

On other flashpoint topics, the pope may well wade in.

He addressed climate change Wednesday, calling it "a problem which can no longer be left to a future generation."

- 'Stick to religion' -

There is expectation Francis will urge lawmakers to give careful consideration to protecting the planet, even as several Republicans in Congress doubt the existence of man-made climate change.

"The pope should maybe stick to religion," Republican congressman Steve King told Politico this week.

"There is nothing in the New Testament about global warming."

The Argentine pontiff waded into another bitter US political debate when he urged the church to embrace new immigrants.

"Perhaps it will not be easy for you to look into their soul. Perhaps you will be challenged by their diversity. But know that they also possess resources meant to be shared," he said.

Francis was cheered by euphoric crowds with breathless wall-to-wall televised coverage as he toured Washington's stately boulevards.

Among the fans was five-year-old Sofia Cruz, US-born but whose parents are from Mexico, who clambered over a barrier, darted out onto Constitution Avenue and caught the attention of Francis -- and of the world.

"She handed the pope a letter asking him to support the drive to legalize undocumented migrants living in the United States," her family's parish in Los Angeles said, contacted by AFP.

More than 11 million people without legal residency, like her parents, could face deportation unless immigration reform is passed.

The visit was a political mirror of pope Benedict's 2008 trip to George W. Bush's White House. Those two leaders were as conservative as their successors are progressive.

Still, the White House insisted it is not co-opting a holy man in order to batter Republican foes in Congress.

"The goal of the pope's visit, and certainly the goal of the meeting was not to advance anyone's political agenda," said White House spokesman Josh Earnest.

The pontiff will address the United Nations Friday in New York, and wraps up his historic six-day US trip Saturday and Sunday in Philadelphia at an international festival of Catholic families.