Trump struck back. No stranger to controversy, the longtime party front-runner in national opinion polls dismissed the leader of the world's Roman Catholics as "disgraceful" for questioning his faith. He said he was a proud Christian.

    Francis told reporters during a free-wheeling conversation on his flight home from a visit to Mexico, "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian."

Trump has accused Mexico of sending rapists and drug runners across the United States' southern border and has 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.