"Both are against life. The one who discards migrants and the one who kills children. Both are against life," Francis told reporters aboard his plane returning to Rome after a 12-day tour of Asia.
"I am not an American and I will not be voting there. But let it be clear: both sending migrants away and not giving migrants the ability to work or welcoming them is a sin, it is serious," he said.
Former president Trump has promised to round up illegal immigrants and deport them as he seeks to return to the White House in the looming November election.
He also paved the way for a 2022 US Supreme Court overturning of Roe v Wade, the 1973 ruling that made abortion a national right for women -- a right that Harris has pledged to restore.
"One has to choose the lesser of two evils. Who is the lesser evil? That lady or that gentleman? I don't know. Everyone have to think and make this decision according to their conscience," Francis said.
