NEW YORK (AP) — Bernie Sanders said Wednesday he would regret refusing an invitation to attend a Vatican conference this week that takes him off the campaign trail just days before the critical New York presidential primary.

Sanders has a 10-minute speaking slot Friday at the conference organized by the Pontifical Academy of Social Sciences, which serves a role at the Holy See similar to a think-tank. Dozens of speakers will commemorate 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.

“I obviously would not be leaving in the middle of a very hard-fought primary here if this were not an opportunity that comes once in a lifetime,” he said in a phone interview. “I am deeply impressed by the commitment of the pope to speak out about economic and social and environmental injustice and I would be kicking myself if I refused this opportunity.”

The chancellor for the pontifical academy, Bishop Marcelo Sanchez Sorondo, has said he invited Sanders because the Democratic senator from Vermont was the only U.S. presidential candidate who showed deep interest in the teachings of Pope Francis. The pope is not expected to attend the conference.

Sanders’ participation in the event comes ahead of primaries in states with large Roman Catholic populations– on Tuesday in New York and on April 26 in Rhode Island and Connecticut.

His trip coincides with a fund-raising trip by rival Hillary Clinton to California, including events hosted by George and Amal Clooney. Sanders said such big-money fund-raising was “part of the corruption of American politics.”

In the Vatican program, Sanders’ talk is titled, “The Urgency of a Moral Economy: Reflections on the 25th Anniversary of Centesimus Annus.”

“What I’m planning to say is that it is not acceptable from a moral perspective, from an economic perspective or from an environmental perspective that so few have so much and that greed is running as rampant as it is throughout the entire planet,” Sanders said.

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