WASHINGTON — Plunging headlong into the issues of the day, Pope Francis opened his visit to the United States with a strong call Wednesday for action to combat climate change, calling it a problem that “can no longer be left to a future generation.” President Barack Obama, in turn, hailed the pontiff as a moral force who is “shaking us out of our complacency” with reminders to care for the poor and the planet.

The White House mustered all the pageantry it had to offer as the pope arrived at the White House before an adoring crowd of thousands and a nation that seemingly cannot get enough of the humble pontiff who is rejuvenating American Catholicism while giving heartburn to some of its conservatives.

Speaking in a soft voice and halting English, Francis delivered a firm message against those who doubt the science of climate change, saying that the warming planet “demands on our part a serious and responsible recognition” of what awaits today’s children.

It was a message sure to please the Obama White House, and liberals in general. But the pope had something for conservatives, too, with a pointed call to protect religious liberties — “one of America’s most precious possessions.”

“All are called to be vigilant,’ he said, “to preserve and defend that freedom from everything that would threaten or compromise it.”

It was a welcome message to many U.S. bishops and conservatives who have objected to the Obama administration’s health care mandate and the recent Supreme Court legalization of same-sex marriage.

With flags snapping, color guard at attention and a military band’s brassy marches, Francis stepped from his modest Fiat onto the South Lawn on a crisp fall morning that felt as optimistic as his own persona. Pope and president stood on a red-carpeted platform bedecked with red, white and blue bunting, standing at attention for the national anthems of the Hole See and the United States.

After their opening remarks on the lawn, Obama and Francis pulled up two arm chairs by the fireplace in the Oval Office for a one-on-one meeting where each hoped to find common cause with the other on issues they hold dear — and respectful disagreement where they differ sharply, on subjects such as abortion and same sex marriage.