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.