Ahead of his trip to the Vatican, Democratic presidential candidate Bernie Sanders said he is a "very great fan" of Pope Francis' support for a moral economy. (Reuters)

Ahead of his trip to the Vatican, Democratic presidential candidate Bernie Sanders said he is a "very great fan" of Pope Francis' support for a moral economy. (Reuters)

Democratic presidential hopeful Bernie Sanders invoked the name of Pope Francis Saturday during a campaign appearance at which he touted his own advocacy of a “moral economy” and challenged Hillary Clinton to endorse his plan to expand Social Security benefits by raising taxes on the wealthy.

Sanders’s prominent mention of the Catholic leader came a day after he disclosed plans to step off the campaign trail next week to attend a conference at the Vatican on income inequality. The trip comes just days before the crucial New York primary, when the senator from Vermont desperately needs a strong showing to maintain momentum in his bid to catch Clinton in the race for the Democratic presidential nomination.

Sanders’s comments Saturday show how he intends to tie the trip into his campaign.

“I must tell you that I am a very great fan of the role that Pope Francis has been playing in talking about inequality in this world,” Sanders told a crowd of about 1,000 in Manhattan after relaying his plans for the trip. “He has been out there talking about the need for a moral economy . . . an economy in which we have a moral responsibility to pay attention to what he calls ‘the dispossessed.’ ”

[Sanders’s side trip to the Vatican poses risks — and potential rewards]

Democratic presidential candidate Bernie Sanders (I-Vt.) sat down with the Salt and Light Catholic Media Foundation where he called Pope Francis a socialist. (Salt and Light Catholic Media Foundation)

Sanders, who is Jewish and describes himself as a democratic socialist, said the test of a great country isn’t how many billionaires it has or the size of its military but rather how it treats “the weakest” of its citizens.

“We must be honest enough to say we are failing that test,” Sanders said.

He then pivoted to talking about expanding benefits for senior citizens struggling to get by on Social Security. Sanders touted his plan to raise taxes on those making $250,000 or more, a move that he said would extend the life of the program and allow for an increase in benefits for those with the greatest need.

“Secretary Clinton really has avoided this issue,” Sanders alleged. “She’s talked about it in generalities.”

His comments brought no immediate response from the Clinton campaign.

Clinton has said that her plans for Social Security and Medicare focus on preventing further cuts, reducing costs and expanding benefits for the poorest recipients — especially widowed and single women.

She has said she would consider lifting a cap on taxable income, as Sanders advocates, and also consider taxing investment income to pay for expanded benefits. But she has not gone as far as Sanders in making those taxes part of her plan.

1 of 46 Full Screen Autoplay Close Skip Ad × Clinton on the campaign trail View Photos Former secretary of state Hillary Clinton visits key states in her quest to become the Democratic nominee for president. Caption The former secretary of state, senator and first lady is the Democratic nominee for president. July 31, 2016 Hillary Clinton is seen aboard the campaign bus in Cleveland on the third day of a bus tour through Pennsylvania and Ohio. Melina Mara/The Washington Post Buy Photo Wait 1 second to continue.

The only reaction to Sanders’s trip from the Clinton campaign has been a message on Twitter from press secretary Brian Fallon, who said “holy smokes” in response to a since-revised report that Sanders invited himself to the Vatican.

Sanders plans to depart for the Vatican almost immediately after a debate with Clinton, a former secretary of state and U.S. senator representing New York, scheduled for Thursday night in Brooklyn.

It remains unclear whether Sanders, who was born in Brooklyn, will meet publicly or otherwise with Francis — an image that could provide a boost to his campaign despite the obvious risks of leaving New York so soon before a primary crucial to his electoral fate.

In a telephone interview, the Rev. Federico Lombardi, the senior Vatican spokesman, said that the invitation to Sanders did not come from the pope and that he was unaware of whether the pontiff had been informed before the offer was extended. Rather, the invitation came via the hand of an influential Argentine cleric in Vatican City, Monsignor Marcelo Sánchez Sorondo, who heads the ancient Pontifical Academy of Sciences.

Sanders is attending a celebration and study session marking the 25th anniversary of Centesimus Annus, a key document on economic and political inequities issued by Pope John Paul II in 1991. “Other personalities of the political, social and economic world have been invited, too,” Lombardi said.

Sorondo, an important figure who helped shape the Vatican’s position on climate change, said in an interview that he invited Sanders because he has been outspoken on some of the issues being discussed at the conference and Sanders has quoted the pope previously. He said he is “very glad” Sanders accepted.

“I would have invited Hillary Clinton, too, if I thought she would accept,” Sorondo said, adding that he did not consult the pope on the Sanders invitation.

Close observers had mixed views of the signals the Vatican was sending.

Massimo Faggioli, a church historian who directs the Institute for Catholicism and Citizenship at the University of St. Thomas, a Catholic school in St. Paul, Minn., tweeted: “If it’s true that Sanders is speaking at a conference in Vatican City next week: it’s a wrong thing [to do] since we’re in the middle of the political campaign.”

[Pope Francis offers hope to divorced Catholics, says no to gay marriage]

Yet Andrea Tornielli, a longtime Vatican watcher and senior member of the Vatican press corps, said he did not perceive an overt political gesture.

“I don’t think they were thinking about the election campaign at the moment Sanders was invited,” he said. “The pope did not invite him, and the meeting is focusing on studying and remembering an important document on economic and political structures. It doesn’t seem aimed at interference in the U.S. campaign.”

He added: “The Vatican does not make these kinds of political statements. If there is a kind of coincidence in the timing, and it’s difficult not to note the timing, I don’t think it was the product of a sustained effort.”

“The pope does make political gestures,” he said. “He is going to Lesbos on Saturday, and that’s a decision to make a statement about Europe and migrants. But he typically does not by choice intervene in domestic political affairs. He avoided going to Argentina [his home country] for two years to avoid the presidential campaign.”

An effusive Methodist, Clinton has portrayed her faith often in this campaign as the foundation of her beliefs, both personal and political.

Clinton has called herself a “great admirer” of Pope Francis and praised him for pushing political forces in Washington and elsewhere toward what she views as a more moral stance on issues such as climate change and poverty.

“I think that what he’s trying to do is take this venerable institution, the Roman Catholic Church, and really, once again, place it on a firm foundation of scriptures of Christ’s words,” Clinton told ABC News in an interview ahead of Francis’s visit to the United States last year. “I think leaders of conscience, particularly leaders of faith, who say what they believe in their heart, and what they are called to say, often make people uncomfortable. And we need that. We need more of that.”

Faiola reported from Berlin. Abby Phillip in New York and Stefano Pitrelli in Rome contributed to this report.