With the first words that Pope Francis uttered publicly on American soil, he made it clear Wednesday that he does not intend to sidestep the deeply divisive political issues that are roiling this country.

But his is the kind of message Washington has not heard in a while. In a city where politics has come to be known for its power to drive people apart, Francis arrived determined to summon them together.

Amid the pomp of a welcoming ceremony that drew 15,000 to the South Lawn of the White House, the first pope from the Americas introduced himself as “the son of an immigrant family” and said, “I am happy to be a guest in this country, which was largely built by such families.”

Coming from the hugely popular and charismatic Hispanic pontiff, that was an unmistakable reference to an issue that has come to dominate much of the race for the 2016 Republican nomination. GOP front-runner Donald Trump has vowed to deport the estimated 11 million immigrants who are in this country illegally and revoke the constitutional guarantee of citizenship to anyone born here.

Francis signaled that he intends to make maximum use of the national spotlight fixed on him for the next four days — and to amplify his influence as the spiritual leader of nearly 80 million Roman Catholics in the United States.

The pope packed a lot into his first full day in the United States. Here's what he did and who he met. (Alice Li and Gillian Brockell/The Washington Post)

On Thursday, he will address a joint meeting of Congress. In the audience will be at least three of the 15 Republican contenders for president.

He was invited by House Speaker John A. Boehner (R-Ohio), a former altar boy who seemed nothing short of star-struck at the prospect of hosting a pope.

“For a little Catholic boy like me, this is big stuff,” Boehner told USA Today.

In his home as Boehner was growing up, “there was a picture of the pope and a picture of President Kennedy,” he said. “And most every day we prayed for the pope as well as the archbishop.”

This particular pope has been more outspoken than most on issues that are not strictly within the realm of doctrine. Democrats were jubilant at what he said Wednesday on immigration, as well as on climate change and on the Obama administration’s opening of diplomatic relations with Cuba, which Francis helped broker.

But as the fourth pope ever to visit the United States has set off on his tour, Francis continues to defy being categorized, either ideologically or as part of a partisan agenda.

Pope Francis addressed 300 U.S. bishops at St. Matthew's Cathedral on Sept. 23. (AP)

The pontiff has brought a softer tone to some of the social issues on which the Catholic Church has played a leading role, such as abortion, divorce and homosexuality. And he has been critical of what he says are the excesses of capitalism.Yet chroniclers of the Catholic Church and the Vatican said they discerned signs that Francis is pushing back on efforts to enlist him in this country’s culture wars, or to distance him from the work that is being done in the political sphere by American bishops.

In a report for the Catholic news site Crux, longtime Vatican watcher John Allen wrote, “Francis has good political radar, and seems to want to reshape the way he’s seen.” His piece was headlined: “Francis 2.0 emerges in America: Pope and church are package deal.”

Francis has not changed any fundamental tenets of Catholic dogma.

In his brief address at the White House, delivered in English, the pope also congratulated President Obama for “proposing an initiative for reducing air pollution. 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.”

Obama’s views and those of the pope on the issue of climate change fall squarely on the liberal side of the political divide. At the same time, some of the pope’s words on other subjects reinforced arguments that are being made by conservatives.

He noted, for instance, that U.S. Catholics are “concerned that efforts to build a just and wisely ordered society respect their deepest concerns and their right to religious liberty. That freedom remains one of America’s most precious possessions.”

The pope specifically noted that a priority of the U.S. bishops has been “to be vigilant, precisely as good citizens, to preserve and defend that freedom from everything that would threaten or compromise it.”

For many American conservatives, the threat has become more urgent with the recent U.S. Supreme Court decision legalizing same-sex marriage, which is anathema to the teaching of the Catholic Church and some other religions.

Francis noted that he will attend the World Meeting of Families in Philadelphia, “to celebrate and support the institutions of marriage and the family at this, a critical moment in the history of our civilization.” Francis has in the past called the rise of gay equality a “new sin against God.”

Thanks in part to the balance he has struck, Francis has created a large space in which politicians of all stripes have been eager to join in the pageantry that has surrounded his visit.

Among the worshipers at the Mass he celebrated Wednesday in Washington were former Florida governor Jeb Bush, a contender for the Republican presidential nomination, and former Maryland governor Martin O’Malley, who is running for the Democratic one.

House Budget Committee Chairman Paul Ryan (R-Wis.) also stood amid hundreds of ticketed guests who awaited security screening and said he planned to send pictures and texts to his family throughout.

“I’m excited about celebrating Mass with the pope today. It’s a great opportunity,” Ryan said. “It’s a very exciting moment as a Catholic to have the pope come.”

And a refreshing one, for a capital where common ground has long been eroding.

There was even a political resonance to the passages of scripture that Francis chose to quote — notably, his invocation of “a city built on a hill” from Matthew.

Americans who don’t recognize the line from the Bible might know it from the 1630 sermon “A model of Christian Charity,” by John Winthrop, a Puritan and founder of the Massachusetts Bay Colony.

It was also a favorite of Ronald Reagan.

Michelle Boorstein contributed to this report.