When two of the most visible figures on the international stage,
The long-distance volley, impelled, like so much of the campaign, by Trump's language on Mexican immigration, created a moment that actually merited the overused label "unprecedented."
Popes have often commented on other countries' politics. But no pope has as pointedly remarked on a central issue in a U.S. presidential campaign as Francis did to reporters on his plane back to the Vatican after his trip to Mexico.
"A person who only thinks about building walls, wherever they may be, and not building bridges, is not Christian," the pope said, in response to a question about Trump's oft-repeated vow to build a wall along the Mexican border. "That is not in the Gospel," he said, according to an English translation of his remarks released by the Vatican.
And no leading presidential candidate has so sharply criticized a major religious figure as Trump did in reply.
"For a religious leader to question a person's faith is disgraceful," Trump said in a statement that he read aloud at a campaign appearance in Kiawah Island, S.C. He also suggested the Mexican government had manipulated the pope, and called himself a "good Christian."
"If and when the Vatican is attacked by ISIS, which, as everyone knows, is ISIS' ultimate trophy, I can promise you that the pope would have only wished and prayed that Donald Trump would have been president," Trump said, using an acronym for the Islamic State militant group.
As with previous controversies involving Trump, the long-distance exchange with Pope Francis dominated the campaign news cycle, drawing attention to him and draining it from rival Republican candidates two days before a critical primary here. In that way, the exchange once again demonstrated the New York billionaire's unrivaled ability to use controversy for maximum political advantage.
"I haven't seen the context of the Holy Father's statement," Rubio, who is Roman Catholic, initially said. Later, in a CNN interview, he defended the idea of a border wall, saying it "is not just about immigrants," but also about "potentially, terrorists crossing that border, not to mention the drugs that are coming across that border and the human trafficking that's occurring."
Former Florida Gov. Jeb Bush, who is also Catholic, told reporters he regarded the pope as his "spiritual leader" but demurred when asked whether Francis was correct to question whether Trump is Christian.
"His Christianity is between him and his creator," Bush said of Trump, who is Presbyterian and has made a show of attending church a handful of times since he began campaigning.
Ohio Gov. John Kasich, speaking at a CNN town hall in South Carolina, said, "Put me down in the pro-pope column."
A member of the Anglican Church in North America, an offshoot of the Episcopal Church, he appeared to side with Trump when he said, "We have a right to build a wall." But, he added, "There are too many walls between us."
Most of Trump's rivals declined to fight with him over immigration, which has proven highly combustible and has formed the center of Trump's campaign. His call for a border wall draws loud cheers. Last week, he began airing a new TV ad claiming credit for forcing other candidates to talk about it.
Moreover, while the pope enjoys widespread popularity in the U.S., his standing is lowest among groups who like Trump the most: Francis has more popularity among Democrats than Republicans and more liberals than conservatives, although majorities in each of those groups view him favorably.
This fall, when the pope visited the U.S., about 8 in 10 Democrats polled said they viewed him favorably, the Pew Research Center found. Among Republicans the share was smaller, a bit more than 6 in 10. Nearly 4 in 10 liberals said Francis had made them feel more positive about the Catholic Church; among conservatives, 2 in 10 said so, the survey found.
Both here and in Nevada, where Trump has begun airing his immigration-themed ad in advance of Tuesday's Republican caucuses, his supporters, both prominent and not, strongly defended him. Some criticized Francis for getting involved.
"Jesus never intended to give instructions to political leaders on how to run a country," said Jerry Falwell Jr., president of Liberty University, who has often called for basing public policy on Christian principles. Falwell, a Trump supporter, made his remarks in an interview on CNN.
"Tell the pope to take a hike," said Mike Price, 61, a schoolteacher attending a rally in this town near the North Carolina border.
"It's none of his business if we want to build a wall," added Price, who was deciding between Trump and Sen. Ted Cruz of Texas.
In Las Vegas, a casino worker who would give only his first name, Jay, because he feared his employer did not want him talking about politics on the job, also proclaimed his support for the candidate.
"I'd vote for Trump. He speaks his mind. Why should he have to apologize?" said Jay, a Republican who was raised Catholic. The pope should stay out of American politics, he said.
"What are you sticking your nose in there for?" he said. "This really isn't your business."
Trump said he expected his remarks and the pope's were "probably going to be all over the world."
"Who the hell cares?" he added. "I don't care. I don't care because we have to stop illegal immigration."
Francis' words came at the end of a six-day tour of Mexico that culminated in a Mass at the border.
Asked about Trump's pledge to build a wall and his insistence on deporting the roughly 11 million immigrants in the U.S. illegally, the pope said he did not intend to meddle in the U.S. election. He would give Trump the benefit of the doubt to determine whether he really said the things attributed to him, he added.
"Whether I would advise to vote or not to vote, I am not going to get involved in that," he said. "I say only that this man is not Christian if he has said things like that."
Trump, characteristically, offered no caveats in his response.
"The Mexican government and its leadership has made many disparaging remarks about me to the pope because they want to continue to rip off the United States," he wrote. "They are using the pope as a pawn."
"No leader," he said, "especially a religious leader, should have the right to question another man's religion or faith."
Bierman reported from South Carolina and Lauter from Washington. Staff writers Tracy Wilkinson in Washington, Seema Mehta in South Carolina and Kate Linthicum and Michael Finnegan in Las Vegas contributed to this report.
A transcript of the pope's remarks was released by the Vatican. Here is the full question and answer:
Q: Today you spoke a lot and eloquently about the problem of immigrants. On the other side of the border there is an electoral campaign that is rather hard. One of the candidates for the White House, Donald Trump, in a recent interview said that you are a political man, and indeed perhaps a pawn of the Mexican Government when it comes to the policy of immigration. He said that if he were elected president he would build a 2,500-km wall along the border. He wants to deport 11 million illegal immigrants and, in that way separating families and so on. I would therefore like to ask, first of all, what you think of those charges against you, and if an American Catholic could vote for a person like this?
A: Thank God he said I am a politician because Aristotle defined the human person as an 'animal politicus' [a political animal]. So at least I am a human person. As to whether I am a pawn, well, maybe, I don't know. I'll leave that up to your judgement and that of the people. And then, a person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian. This is not in the gospel. As far as what you said about whether I would advise to vote or not to vote, I am not going to get involved in that. I say only that this man is not Christian if he says things like that. We must see if he said things in that way and in this I give the benefit of the doubt.
For more on the 2016 campaign, follow @DavidLauter