Pope Francis has been invited by House Speaker John Boehner to speak to a joint session of Congress.

Boehner extended the formal invitation today in a letter to the Vatican - on the one-year anniversary of his papacy - saying the Holy Father has "awakened hearts on every continent.".

"Pope Francis has inspired millions of Americans with his pastoral manner and servant leadership, challenging all people to lead lives of mercy, forgiveness, solidarity, and humble service," Boehner wrote. "His tireless call for the protection of the most vulnerable among us - the ailing, the disadvantaged, the unemployed, the impoverished, the unborn - has awakened hearts on every continent."

Boehner, a Catholic, said the pope's message "challenges people of all faiths, ideologies and political parties."

Since Paul VI made his first papal visit to the United States in 1965, three popes have come to America. John Paul II, who came to the United States seven times, visited with every president from Jimmy Carter to Bill Clinton.

A pope has visited the United Nations four times, but one has never addressed the House and Senate during a joint session.

House Minority Leader Nancy Pelosi, also a Catholic, joined the speaker in his invitation.

"Whether inspired by St. Francis of Assisi, who cared for all of God's creation, or by St. Joseph, protector of the church," Pelosi said, "Pope Francis has lived his values and upheld his promise to be a moral force, to protect the poor and the needy, to serve as a champion of the less fortunate, and to promote love and understanding among faiths and nations."

Congressional officials told ABC News they did not know whether Pope Francis would accept the invitation to visit Washington. It is an open-ended invitation, aides said.

"His social teachings, rooted in 'the joy of the gospel,' have prompted careful reflection and vigorous dialogue among people of all ideologies and religious views in the United States and throughout a rapidly changing world," Boehner said, "particularly among those who champion human dignity, freedom, and social justice. "

House Speaker John Boehner's Invitation to Pope Francis