Bernie Sanders (2nd row down, center) liked what he heard. REUTERS/Jonathan Ernst

Sen. Bernie Sanders (I-Vt.) was the lone Democratic presidential hopeful -- and the only Jewish contender -- in the House chamber for Pope Francis's speech. When he left, he was beaming, as the pope had cited an American Catholic whom Sanders had plenty of praise for.

"The name Dorothy Day has not been used in the United States Congress terribly often," said Sanders in a short interview. "She was a valiant fighter for workers, was very strong in her belief for social justice, and I think it was extraordinary that he cited her as one of the most important people in recent American history. This would be one of the very, very few times that somebody as radical as Dorothy Day was mentioned."

Pope Francis says Abraham Lincoln, Martin Luther King Jr., Dorothy Day and Thomas Merton embody the spirit of America. (AP)

Day was arguably the least famous of the four Americans Francis cited in the speech, but she was a vital and controversial figure on the American left. Like Martin Luther King -- also cited by Francis -- she had a thick FBI file, compiled by agents tracking her support for democratic socialism and opposition to foreign wars. "We need to change the system," Day wrote in 1956. "We need to overthrow, not the government, as the authorities are always accusing the Communists 'of conspiring to teach [us] to do,' but this rotten, decadent, putrid industrial capitalist system which breeds such suffering in the whited sepulcher of New York."

[Who were Dorothy Day and Thomas Merton?]

Recent support for Day's possible canonization has angered some conservatives. "Vatican archives are filled with reports of Christians martyred under the regimes that Dorothy Day supported," wrote Virginia State Senator Dick Black in a 2013 letter to Francis's predecessor, Pope Benedict. "I am revolted by the United States Conference of Catholic Bishops’ support for the canonization of a woman whose views supported the violent extermination of Christians throughout the world."

In Pope Francis' address to Congress, he praised Dorothy Day, founder of the Catholic Worker Movement for her "passion for justice." The Rev. James Martin explains in this excerpt from "Dorothy Day: Who Cares About the Saints?" (The Rev. James Martin and Loyola Productions)

Sanders, well aware of Day's views and her critics, considered it bold and telling that Francis would praise her. "He is willing to identify with an extraordinarily courageous woman whose life was about standing with the poorest people in America, and having the courage to stand up to the very powerful," he said. "You know, her newspaper was the Catholic Worker, and she stood with the workers of America and fought for justice."

The Vermont senator was less concerned by Francis's glancing mentions of other issues that animate progressives, such as climate change and gay rights. "He knew where he was speaking," said Sanders. "I think he does not want to be rude, as a guest. But I think his calling out for social justice, his talking about income and wealth inequality, his talking about creating an economy and a culture that works for everybody, not just a few, is a very, very powerful message."