Democratic presidential candidate Bernie Sanders arrived in Rome on Friday to attend a Vatican conference steeped in his lifelong passions of economic and social justice, briefly departing New York just days before its critical primary to show solidarity with the teachings of Pope Francis.

Sanders made the trip hours after Thursday night's Brooklyn debate with rival Hillary Clinton and said the opportunity to address the Vatican conference was too meaningful to pass up. The roughly 24-hour visit will precede a crucial New York primary against the former secretary of state on Tuesday.

Sanders said in an interview with The Associated Press that he was "appreciative and proud" to be invited to the conference organized by the Pontifical Academy of Social Sciences.

He will join several speakers commemorating the 25th anniversary of "Centesimus Annus," a high-level teaching document by Pope John Paul II on the economy and social justice at the end of the Cold War.

"The theme from the conference, which is essentially how we create a moral economy, is one that has occupied my attention for decades. And the teachings from Pope Francis have moved me very much," the Vermont senator said.

Sanders was working on his speech during the flight and was accompanied by his wife, Jane Sanders, and 10 family members, including four grandchildren.

His talk is titled "The Urgency of a Moral Economy: Reflections on the 25th Anniversary of Centesimus Annus." The conference was organized by a Vatican advisory group comparable to a think tank that Francis appointed to guide him on a wide range of public policy issues.

Clinton holds a significant delegate lead against Sanders, but the senator has vowed to stay in the campaign until the party's July convention. His message calling for a political revolution to address wealth inequality and the influence of Wall Street on U.S. politics has galvanized many Democrats and independents.

A Sanders rally Wednesday night in New York City drew 27,000 people.

Despite being enmeshed in an increasingly bitter campaign against Clinton, Sanders aides said the trip was not aimed at appealing to Catholic voters who comprise a large share of the Democratic electorate in New York and an upcoming contest in Pennsylvania.

"This is not going to be a political speech," Sanders senior adviser Tad Devine said. "We're not looking at this through a political lens."

Sanders, the first Jewish candidate to win a presidential primary, called the trip "an opportunity that comes once in a lifetime."

Sanders and his campaign had held out hope that he might meet with the pope during his visit. Francis will be in Rome on Friday, but the Vatican said he had no plans to either address the conference or meet with Sanders.

The Vatican has been loath to get involved in electoral campaigns and usually tries to avoid any perception of partisanship involving the pope. Popes rarely travel to countries during the thick of political campaigns, knowing a papal photo opportunity with a sitting head of state could be exploited for political ends.

As a result, the invitation to Sanders to address the Vatican conference raised eyebrows and allegations that the senator lobbied for the invitation.

The chancellor for the pontifical academy, Bishop Marcelo Sanchez Sorondo, has said he invited Sanders because he was the only U.S. presidential candidate who showed deep interest in the teachings of Francis.

Other attendees will include Presidents Evo Morales of Bolivia and Rafael Correa of Ecuador, along with Cardinal Oscar Rodriguez Maradiaga of Honduras, a member of the academy, and Columbia University professor Jeffrey Sachs, an adviser to the United Nations on environmental and sustainability issues. Sachs has advised Sanders on foreign policy issues.

Morales met for about a half-hour Friday morning with Francis before heading into the conference. Morales raised eyebrows with an unusual gift for the pontiff: three books about the health benefits of the coca leaf, the raw ingredient for cocaine.

The Rev. Matt Malone, editor of the Jesuit magazine America, said Sanders' trip was unlikely to have much of an impact on Catholic voters, noting that conferences like the one Sanders is attending "happen all the time."

"I don't think that Bernie Sanders going to the Vatican is going to help Bernie with Catholics any more than Ted Cruz going to a matzo factory is going to help him with the Jewish vote," said Malone, who served as a speechwriter to former Rep. Marty Meehan, a Massachusetts Democrat.

Popes rarely attend such events and do so only if the topic is of special interest and there is room in their schedule, Malone said.

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Zoll reported from New York. Associated Press writer Nicole Winfield in Vatican City contributed to this report.