Democratic presidential candidate Bernie Sanders speaks after being endorsed by the Transportation Workers Union Local 100 in Brooklyn on Wednesday. His Vatican trip looms. (Brian Snyder/Reuters)

Sen. Bernie Sanders would have forever regretted skipping the chance to speak at a Vatican conference Friday and, perhaps, to meet Pope Francis, he said in an interview Wednesday.

Sanders’s decision to leave New York just days before a crucial primary here against Democratic front-runner Hillary Clinton — for a 15-minute speech and no guarantee of a direct audience with the pontiff — has prompted much speculation about the self-described democratic socialist’s apparent infatuation with Francis as well as his commitment to the presidential campaign.

But even with the potential for dire political consequences — Sanders is vacating the New York campaign days before what has become a watershed contest here that he needs to win to sustain his argument that he can still capture the Democratic nomination — the senator from Vermont said he simply could not pass it by.

“I would be kicking myself forever if I did not seize the opportunity,” Sanders said in a telephone interview Wednesday.

Along with its possible downside, the gamble also comes with a potentially enormous payoff: a photograph with the pope that would be seen around the world.

Democratic presidential candidate Bernie Sanders is taking two days off from campaigning to attend Pope Francis's conference on social, economic and environmental issues in Vatican City. Here are five things those men share. (Sarah Parnass/The Washington Post)

“It’s something I would be very proud to see happen,” Sanders said of an audience with the pontiff. “I believe that the pope has been an inspirational figure in raising public consciousness about the kind of income and wealth inequality we are seeing all over this world.”

The decision to head to the Vatican even came as a surprise to one of Sanders’s closest advisers, who admitted last week that he played no role in the planning. But for Sanders, there is no mystery in accepting a chance to meet the pope, whose pronouncements on economic justice and environmental conservation he has long admired.

Sanders said the trip is worthwhile whether he gets the audience or not. “It’s absolutely worth the trip to participate in a conference under a pope who has led the world in raising consciousness on the most important issues of social, economic and environmental justice.”

Sanders will be away from New York less than two full days — perhaps no longer than Clinton will spend on a fundraising jaunt to California that will stretch over Friday and Saturday. And the trip could make Sanders appear more presidential, particularly if he secures an audience with the pontiff.

Some politics and theology experts speculated that the trip looks to be more about Sanders’s credentials as the leader of a movement than his qualifications for the White House. Meeting Francis, the hugely popular pope whose advocacy of a more moral economic system has made him a worldwide folk hero, could confer on Sanders greater international stature and move him beyond the confines of the American political race.

“Even a photo op with the pope and a discussion and conversation about these issues puts Sanders on the level of a world leader and reinforces the image that he’s a world leader,” said Costas Panagopoulos, a Fordham University professor of political science who currently is a visiting professor at Yale.

“He’s not making the trip to kiss the ring, but that’s effectively what he is going to do,” Pana­gopoulos said. “It’s a blessing that at the most basic level suggests that Sanders is qualified and competent to address these issues” on the world stage.

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)

Chad C. Pecknold, a theology professor at Catholic University who specializes in U.S. politics, said that the trip may also lay some groundwork for a larger — even international — role for Sanders after an election that is looking to be increasingly difficult for him to win.

“Let’s say that he is realistic and that his path to the nomination is extremely slim,” Pecknold said. “Then he must say, ‘Well, what is my role after this campaign?’ It seems clear that he is in a position now to highlight his new profile.”

Tad Devine, Sanders’s chief strategist, declined to discuss any political implications of the trip during an interview Wednesday.

“It’s not going to be a political speech or event,” Devine said. “We’re not viewing this through a political lens.”

Campaign manager Jeff Weaver said the candidate’s trip is “beyond politics.” The subject of the conference is “the work of Bernie Sanders’s life,” Weaver said.

Mo Elleithee, director of Georgetown University’s Institute of Politics and Public Service, said that he is not sure Sanders’s choice indicates anything about his future.

“I think he’s earned the right to be viewed as a serious presidential candidate,” Elleithee said. “We’re past the point where his campaign is a way to raise the profile of an issue.”

The trip is unusual by many measures. Devine was not consulted and initially seemed startled by Sanders’s sudden announcement last week that he would go.

In addition, the invitation set off a public spat among the organizers of the academic conference he will attend; one accused Sanders of improperly inviting himself, while another said that he had extended the invitation to Sanders himself.

There has also been much speculation about whether the visit will improperly draw Francis into U.S. politics. It wouldn’t be the first time; after declaring he would stay out of the U.S. presidential campaign, the pope offered some harsh words for Republican front-runner Donald Trump.

“This speaks volumes about the lack of coherence of this pope,” said Sandro Magister, a longtime Vatican reporter for the L’Espresso weekly magazine in Italy, “because you can’t logically say ‘I’m not getting mixed up’ and then at the same time disqualify a candidate while welcoming with full honors another one.”

U.S. political candidates do not often break off for foreign travel in the midst of a fierce primary — especially when trailing by double digits in a crucial contest.

“I think it’s a very odd decision that takes him off the campaign trail at the worst possible time,” said Tommy Vietor, a former national security spokesman for President Obama who has advised Clinton.

“Even with extensive planning, international trips are dicey for candidates — just ask Mitt Romney,” Vietor said, referring to the 2012 Republican nominee’s missteps during a poorly reviewed campaign trip to Britain and Israel.

Sanders is scheduled to address a small Vatican conference on income inequality, his signature issue in the American campaign.

While prestigious for its role in informing papal statements about peace and justice issues, the organizing body is two bureaucratic steps removed from Francis. The pope would not have been involved in selecting the speakers, according to theologians and Vatican watchers.

The trip was planned to take Sanders out of New York for as little time as possible. He is departing shortly after Thursday night’s Brooklyn debate against Clinton, and is scheduled to return by early Saturday afternoon, in time to hit the campaign trail later the same day.

In other words, he could spend less than 20 hours on the ground in Rome and at the Vatican. No other events have been announced beyond his 15-minute speech at the conference.

“It’s curious timing, but at the end of the day, does it hurt him? I don’t know,” said Elleithee, who was an aide to Clinton during her 2008 presidential bid. “The venue is one that’s going to get him a lot of attention, so I bet people in New York are going to hear about it.”

Sanders’s Vatican trip is being bracketed by some large-scale rallies intended to make a statement about his drawing power in New York.

Sanders addressed an estimated 27,000 people Wednesday night in Manhattan’s sprawling Washington Square Park, where his speech was repeatedly disrupted by waves of cheers of “Bernie! Bernie! Bernie!” He was joined by an entourage of celebrities, including director Spike Lee, actresses Rosario Dawson and Shailene Woodley and actor Tim Robbins. The band Vampire Weekend played.

Paul Song, the executive chairman of the progressive Courage Campaign, told the crowd that his family was split between support for Clinton and Sanders. He called on “corporate Democratic whores” to be defeated and replaced by “Berniecrats.” Later, on Twitter, Song apologized for the impression that he was insulting Clinton.

Sanders has large rallies planned for Sunday and Monday ahead of Tuesday’s primary.

Clinton, who represented New York for eight years as a U.S. senator, is also campaigning almost daily in the state.

Sanders’s aides have argued that Clinton is probably spending more time out of state than Sanders, given fundraising trips to Florida, Virginia, California and elsewhere. Sanders, who raises the vast majority of his money online, generally has the luxury of spending more time than Clinton doing public campaign events.

Sanders, a secular Jew, refused to speculate about whether the trip to the Vatican might boost support among Catholic voters.

“Let’s be clear, I think Pope Francis is an extraordinarily beloved figure all over this world,” Sanders said. “The love that is being shown to him has gone far beyond the Catholic Church. He is enormously popular in the United States. . . . I think he has struck a nerve. I’m just enormously grateful for the role he has played.”

Zauzmer reported from Washington. Stefano Pitrelli in Rome contributed to this report.