Wading into one of the most polarizing issues in American politics, Pope Francis has called upon the United States to join other nations in fighting climate change.

He stressed the time to act is now. "Climate change is a problem which can no longer be left to a future generation. When it comes to the care of our common home, we are living at a critical moment of history," the pope said in a speech Wednesday on the South Lawn of the White House.

There were nearly 15,000 people in the audience, many of them Republican lawmakers who are skeptical about the need for a radical action on global warming.

While he commented on other issues like immigration, the pope devoted a great part of his speech to climate change. He invoked the spirit of Martin Luther King, Jr. in sending his message across. "To use a telling phrase of the Reverend Martin Luther King, we can say that we have defaulted on a promissory note and now is the time to honor it."

King, in his famous "I Have a Dream" speech in 1963, deplored the treatment of African-Americans, saying the country had defaulted on a promise to grant all men inalienable rights.

The pope commended President Barack Obama for his efforts to tackle climate change. "Mr. President, I find it encouraging that you are proposing an initiative for reducing air pollution."

That was in reference to the Obama administration's new proposed climate change regulations under the Clean Power Plan. The initiative, which is facing opposition from Republicans, aims to cut carbon dioxide emissions by 32% from 2005 levels by 2030.

The spiritual leader of more than 70 million Catholics in the world's richest nation made clear his view that protecting the environment was a moral issue.

"I would like all men and women of goodwill in this great nation to support the efforts of the international community to protect the vulnerable in our world and to stimulate integral and inclusive models of development, so that our brothers and sisters everywhere may know the blessings of peace and prosperity which God wills for all his children," he said.

Francis has been a critic of unfettered capitalism and the harm it does to the environment. Four months ago, he issued an encyclical to the church embracing environmental protection as one of his major advocacies as pope.