WASHINGTON -- For all its savoir faire about visiting heads of state and royalty, the nation's capital is exhibiting a greater degree of anticipation than usual for Pope Francis and his historic address to Congress as he begins his three-day sojourn here Tuesday.

Beyond how to cope with massive crowds, heightened security and traffic gridlock, the question being asked most often is: What message will Pope Francis deliver Thursday as the first pontiff to speak to a joint meeting of the House and Senate?

Pope Francis' address, which he will read in English and is expected to last 20 minutes, will have political ramifications, with high stakes both for the pontiff delivering it and the lawmakers listening, scholars say.

See alsoComplete coverageInteractiveYour messages to the popeSee alsoPope's visit: Follow along at News 12

White House aides and House Speaker John Boehner (R-Ohio) -- who succeeded in inviting a pope to speak to Congress after trying for 20 years -- said they're unsure exactly what the pontiff will say.

Catholic leaders and scholars offer vague predictions.

Cardinal Donald Wuerl, archbishop of Washington, on Fox News Sunday said, "It will be a pastoral speech" that could echo the encyclical on climate change and on "the dignity of every human person, the value and sanctity of life, but also on the social development that allows a life to fully develop."

advertisement | advertise on newsday

Stephen Schneck, director of Catholic University of America's Institute for Policy Research and Catholic Studies, said: "It won't fit into any of the polarized boxes of politics in America. Progressives and conservatives aren't going to like everything he has to say."

In his first visit to the United States, Francis will address 80 million U.S. Catholics and a Congress with a greater presence of Catholics than the population.

The charismatic pope might look for a balance in his words, given the divide over some of his recent statements and actions.

Get the Newsday Now newsletter!

The best of Newsday every day in your inbox.

Younger and Latino followers find him inspiring, but conservatives soured on him after he spoke of climate change and the "idolatry of money," a July Gallup Poll found.

Rep. Paul Gosar (R-Ariz.), for example, said he's skipping the speech because "to promote questionable science as Catholic dogma is ridiculous."

When Pope Francis arrives at Andrews Air Force Base Tuesday afternoon, President Barack Obama will bestow the rare honor of picking him up.

Obama has shared similar views with the pontiff on climate change, income inequality and restoring U.S.-Cuba relations. The pope has called on the world to accept more refugees fleeing Syria, and U.S. officials have agreed to hike the number they will accept.

The GOP-led Congress has raised traditional values: The House voted last week to strip Planned Parenthood's federal funds because it performs abortions, and the Senate will hold a vote the day the pope speaks.

advertisement | advertise on newsday

But the speech is just one item on a schedule that will draw immense crowds requiring security and closed streets.

A White House welcoming ceremony Wednesday will include 15,000 guests. Later that day, thousands will view his tour of the Ellipse and National Mall, and 25,000 are expected to attend the outdoor Mass at the Basilica of the National Shrine of the Immaculate Conception for the canonization of 18th century friar Junipero Serra.

On Thursday, 50,000 people will be on the Capitol's west lawn during the address.

Francis has generated excitement among Catholics and others because of "the unique role that he has carved out as a substantial moral voice on the world stage," White House national security aide Ben Rhodes said. "That has resonated with audiences in a way that, frankly, other world leaders have not."The pope is expected to be candid no matter where he goes, Schneck said.

"It's the off the cuff remarks" that have drawn so much attention, he said. "We don't really know what's going to come. I get goose bumps thinking about the week ahead."