Pope Francis forcefully injected himself into the U.S. presidential campaign on Thursday, assailing Republican candidate Donald Trump's views on U.S. immigration as "not Christian" in a sign of growing international concern at the billionaire businessman's prospects of winning the election.
Trump struck back, swiftly dismissing the leader of the world's Roman Catholics as "disgraceful" for questioning his faith.
"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis told reporters during a free-wheeling conversation on his flight home from a visit to Mexico.
No stranger to controversy, Trump, the longtime party front-runner in national opinion polls, has accused Mexico of sending rapists and drug runners across the United States' southern border and vowed if elected president to build a wall to keep out immigrants who enter illegally.
It was not the first time U.S. allies have voiced concern over comments by Trump.
More than half a million Britons signed a petition to bar him from entering the country, where he has business interests, in response to his call to ban Muslims from entering the United States. British lawmakers decided against a ban as a violation of free speech.
It was not clear what impact the pope's remarks would have on the Nov. 8 presidential election.
Asked if American Catholics should vote for someone with Trump's views, Francis said, "I am not going to get involved in that. I say only that this man is not Christian if he has said things like that. We must see if he said things in that way and in this I give the benefit of the doubt."
It remained to be seen if the pope's comments would strengthen Trump, whose swipes at rival candidates and heated exchanges with others have bolstered his standing in nominating contests and opinion polls.
Republican presidential candidate Jeb Bush, speaking in Columbia, South Carolina, said he would not question anyone's relationship with God. But Bush, a Catholic, said, "It only enables bad behavior when someone from outside our country talks about Donald Trump."
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Trump, a real estate developer and former reality TV show host, said, "If and when the Vatican is attacked by ISIS, which as everyone knows is ISIS's ultimate trophy, I can promise you that the pope would have only wished and prayed that Donald Trump would have been president."