Democratic presidential candidate Bernie Sanders has arrived in Vatican City to speak at an economy and social justice conference.
There are no plans for him to meet personally with Pope Francis.
“I’m so excited and proud to be here with other people who are trying to create a moral economy,” he told supporters outside the Vatican.
The visit comes just four days before a crucial state primary contest in his native New York.
On Thursday night, the Vermont senator and his rival Hillary Clinton clashed on Wall Street banks, gun controls and the minimum wage in a punchy debate.
Afterwards he flew to Rome, where supporters were gathered at the Vatican wearing T-shirts saying: “Rome feels the Bern.”
Mr Sanders said inequality, injustice and pollution are causing “devastating” problems.
And he admires the pope for his views on income inequality and greed.
Some political commentators have wondered why he would travel to Rome so close to the New York primary.
“It’s a real misstep,” New York Times columnist Charles Blow said on CNN. “He’s basically going to be saying the same thing there that he’s been saying here.”
Mr Sanders’ campaign manager Jeff Weaver defended the trip at a rally.
“Hillary Clinton has been out of the state on a number of occasions for high-dollar fundraisers,” he said. “We think this is a more important reason to leave the campaign trail for the day.”