Yesterday, Pope Francis added the strongest voice yet to a growing chorus of world leaders taking a stand against the celebrity candidate - condemning Trump's hard-line immigration agenda and suggesting he was not a Christian because of it.
In taking the rare step of injecting his views into the United States campaign, the Pontiff made remarks that underscored the anxiety coursing through world capitals about a possible Trump presidency. Francis noted Trump's promise to deport an estimated 11 million undocumented immigrants living in the United States illegally and make Mexico pay for a wall along the border to keep them out.
"This is not in the Gospel," he added.
The Pontiff's remarks jolted the Republican contest before tomorrow's crucial South Carolina primary, immediately overshadowing the closing arguments the top candidates were making on the campaign trail.
Trump, a Presbyterian, strongly rebuked Francis' comments, seeking to gain the upper hand politically in a state where polls show him with a double-digit lead.
"For a religious leader to question a person's faith is disgraceful," Trump said at a campaign rally in Kiawah Island. "I'm proud to be a Christian, and as president I will not allow Christianity to be consistently attacked and weakened, unlike what is happening now with our current President."
Trump's immigration positions have been at the core of the billionaire mogul's emotional and nativist pitch to voters - and many of his supporters rallied to the candidate's defence in the face of the Pontiff's criticism.
As Trump sat down for lunch at Fratello's Italian Tavern in North Charleston, Mayor R Keith Summey reassured the candidate.
"I don't care if he's the Pope or not, you know, the bottom line is: Your faith is your faith," said Summey, who had just endorsed Trump. "I'm a Baptist, but I think there are some darn good Catholics and darn good Jews, a lot of good people out there in this country, and just because I say something they disagree with doesn't mean that I have any less faith than they have."
Trump replied: "It's a very sad situation." Though minutes later, Trump told reporters he remained "totally respectful" of the Pope.
For Trump, who had been spending the week on the defensive over his dramatic changes on social issues, Francis' comments helped him redirect attention to immigration. There also were potential long-term risks for Trump's candidacy, however, considering the Pontiff's wide popularity across the country and the respect he commands.
"The Pope was in Mexico," he said at his rally. "Do you know that? Does everyone know that? He said negative things about me because the Mexican Government convinced him that Trump is not a good guy because I want to have a strong border, I want to stop illegal immigration, I want to stop people from being killed."
For generations, the role of the papacy has been shaded by politics - and Francis especially, with his efforts on global climate change and general outspokenness. But it was striking for him to comment on US electoral politics during an active campaign.
"He has a general duty to remind us of our Christian obligations, but attaching them to politically partisan proposals is unwise. One has to question these off-the-cuff remarks that make the Pope look like someone who's grinding an ax," said John O'Sullivan, a historian of papal politics and the president of the Danube Institute in Budapest.
Francis' commentary was the latest evidence of intense global interest in the 2016 presidential campaign - especially Trump's unpredictable candidacy.
"The presidential campaign here is of intense interest to the entire world, in no small part because people and countries everywhere will be affected by our choice," said Richard Haass, president of the Council on Foreign Relations.
Last month in London, after Trump proposed temporarily banning all Muslims from entering the US, members of Parliament debated whether to ban Trump from travelling to the United Kingdom. In the House of Commons, lawmakers described him with a flurry of insults: an "idiot", a "buffoon", a "demagogue" and a "wazzock". British Prime Minister David Cameron had earlier called Trump's travel ban on Muslims "divisive, stupid and wrong". And London Mayor Boris Johnson said that Trump was "out of his mind" and "unfit" to be president.
"The only reason I wouldn't visit some parts of New York is the real risk of meeting Donald Trump," Johnson said.
Similar condemnations have rained in from around the globe. French Prime Minister Manuel Vals has accused Trump of "feeding hatred and misinformation". Turkish President Recep Tayyip Erdogan has said of Trump's Muslim ban, a "politician shouldn't talk like this". And Saudi Prince Alwaleed bin Talal took to Twitter to call Trump a "disgrace to America". On the ground in South Carolina, however, Trump's politically incorrect statements and policy pronouncements have propelled him to the pole position heading into tomorrow's primary. Many of his supporters scoffed at the Pope's comments.
Pam Ridgway, a 64-year-old retired teacher from North Charleston who saw Trump eat lunch, said she was "really surprised" by the Pope's comments.
"You can't have a country unless you have borders, and evidentially we can't have borders if we don't have the wall because we're not taking very good care of our borders," said Ridgway, who said she plans to vote for Trump. "And I think he's a Christian, if he says he is."
Later in the day at a Trump rally event in Gaffney, Walter Lansford of Boiling Springs said that "Trump's right" in the spat with the Pope.
"Even the liberals that we talk to, they're liberal to a point. They're willing to let 13 million illegals stay here, but they don't want 150 million more to come," said Lansford, 69. "So the Pope has his opinion, Trump has his. I don't have a problem, I just support everything Trump has said so far."
Most of the other Republican candidates handled Trump's clash with Francis cautiously, giving a series of non-answers that revealed their fear of inflaming either side considering the political ramifications of the dispute were not immediately obvious.
Senator Ted Cruz said in Easley, "That's between Donald and the Pope ... I'll leave it to the two of them to work out." Without directly responding to Francis's remarks, Senator Marco Rubio said the US has "a right to control who comes in, when they come in and how they come in". "Vatican City controls who comes in, when they come in and how they come in as a city-state," Rubio, who is Catholic, said during a stop in Anderson.
Campaigning in Columbia, former Florida Governor Jeb Bush, who is also Catholic, said it was improper for the Pope to question Trump's faith.
"I think his Christianity is between him and his creator," Bush told reporters.
Ohio Governor John Kasich took a different approach, however, heaping praise on Francis when asked about the pope's Trump critique. "I love the Pope," Kasich said following a campaign event in Clemson. "The Pope, in terms of his overall message, has been one of love and compassion ... I'm not even sure I'm qualified to criticise or comment on remarks from this man."
Five papal feuds with political leaders
Pope John Paul II vs Philippines President Ferdinand Marcos, 1981: In a speech with President Marcos sitting nearby, the Pope explicitly criticised the Marcos regime, saying, "Even in exceptional situations that may at times arise, one can never justify any violation of the fundamental dignity of the human person or of the basic rights that safeguard this dignity." It was Marcos' turn to speak next; he apparently apologised to the Pope for "petty and small" conflicts between the church and state.
Pope John Paul II vs former Nicaraguan Sandinista leader Miguel D'Escoto Brockmann, 1985: In the 1980s, a leading Nicaraguan priest became involved in the country's political revolution, eventually helping lead the left-wing Sandinistas. Pope John Paul II publicly reprimanded Miguel D'Escoto Brockmann, and the Vatican suspended him and two other priests for their involvement. In 2015, an 81-year-old D'Escoto asked the Vatican to reinstate him so he could celebrate Mass again "before dying". Under Pope Francis, he was welcomed back into the church.
Pope John Paul II vs Chilean President Augusto Pinochet, 1987: While speaking to reporters aboard a plane en route a two-week South American tour, John Paul II said Chile's Pinochet-ruled Government was "dictatorial". Pinochet ran Chile from the 1970s to 1990 and presided over thousands of politically linked deaths, imprisonments and disappearances. The New York Times said that asked if he expected to help bring democracy to Chile, the Pope said: "Yes, yes, I am not the evangeliser of democracy, I am the evangeliser of the Gospel. To the Gospel message, of course, belongs all the problems of human rights, and if democracy means human rights it also belongs to the message of the church."
John Paul II vs Bill Clinton, 1993-1994: Pope John Paul II and Clinton clashed publicly several times on the issue of abortion. While visiting the US in 1993, the Pope and Clinton met for the first time. The two said they had a warm private meeting, but then in public remarks, with Clinton standing by his side, the Pope criticised the President for his abortion-rights stance. A year later, the two had another open disagreement at a United Nations summit in Cairo about abortion and the availability of birth control.
Pope Benedict XVI vs Turkey, 2000s: A year before he became pope in 2005, Benedict told French daily newspaper Le Figaro that Turkey shouldn't be allowed into the European Union in part because of the number of Muslims in the country. In 2006, journalists accompanying Benedict on a trip to Turkey reported that he apparently tried to walk back those comments. But later, the Pope said Turkey's admittance into the EU must be dependent on religious freedom. "In every step towards unification, minorities must be protected."
Comments strike US Christians too
After Pope Francis suggested yesterday that Donald Trump "is not Christian" because he focuses so intently on a border wall and the mass deportation of millions of illegal immigrants, Trump supporters were quick to note that Francis' home, the Vatican, is fortified by - wait for it - border walls!
Which is not a bad retort.
This tweet is from Trump's social media director, Dan Scavino: "Amazing comments from the Pope - considering Vatican City is 100 per cent surrounded by massive walls."
But as to the substance of the Pope's comments, there's something else that many a Christian will notice. And that's the fact that he basically said that something many of them support is "not Christian".
A November Washington Post-ABC News poll asked whether people wanted to deport "all" of the estimated 11 million undocumented immigrants in the United States. And Christians were very evenly split, with 46 per cent in favour and 52 per cent opposed. (Catholics were slightly less in favour, with 37 per cent supporting that particular policy.) A January Post-ABC poll, meanwhile, showed 42 per cent of Christians said immigrants from other countries weaken the US. The Pope didn't weigh in on this particular idea, but his philosophy is generally much more pro-immigrant than that.
Finally, while we don't have good data on how many Christians favour a border wall, Americans overall are pretty split, with 45 per cent in favour and 49 per cent against, according to a January CBS News/New York Times poll.
And given the fact that Christians tilt more conservative than the population overall on almost every issue, it's a good bet that half or more of Christians support a border wall.
Now, it's important to note here that the Pope didn't just cite these policies - he cited "a person who thinks only about" border walls and not "building bridges". So you could make the case that his criticism applies more to Trump than to other Christians, given Trump's almost-singular focus on a border wall and deportation, along with his divisive rhetoric.
But it's also true that, for many conservative Christians in the US, the immigration debate starts and ends with a border wall and deportation. That's basically the operating principle of today's heavily Christian Republican Party, in fact.
In other words, Donald Trump is hardly the only person in America who is probably taking exception to Pope Francis' comments.