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.