Pope Francis says a brief meeting with Democratic presidential candidate Bernie Sanders just "good manners" and not political interference.
They met early in the lobby of the Pope's residence before he left to meet migrants on the Greek island of Lesbos.
"If anyone thinks that greeting someone is getting involved in politics, I recommend that he look for a psychiatrist," Pope Francis added.
For his part Mr Sanders said it was an "honour" to meet him.
"I think he is one of the extraordinary figures not only in the world today but in modern world history," he told Associated Press news agency.
"I told him that I was incredibly appreciative of the incredible role that he is playing in this planet in discussing issues about the need for an economy based on morality, not greed," Mr Sanders added.
He had been in the the Vatican to attend a conference on economy and social justice and stayed at the same guest house where the Pope lives.
On the meeting Pope Francis told reporters:
"This morning when I was leaving, Senator Sanders was there, he had come to the convention. He knew I was leaving at that time and he had the courtesy to greet me. I greeted him, his wife and another couple who were there and were sleeping in Santa Martha.
"When I came down, I greeted him, I shook his hand and nothing more. This is called good manners and it is not getting involved in politics."
Late on Friday Mr Sanders fulfilled his promise to release his tax return, which showed that he and his wife Jane had taxable income of $205,000 (£144,000) in 2014, the majority of it from his Senate salary.
This puts him almost in the top 5% of US earners, Reuters news agency says.
His opponent, Hillary Clinton, and her husband Bill have earned nearly $140m over the past eight years, according to previously released tax returns.
Some political commentators have wondered why he would travel to Rome so close to Tuesday's crucial 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's 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."
Mrs Clinton, who represented the state in the US Senate for two terms, holds a commanding lead in New York, according to recent polls.