NEWBERRY, S.C. (Reuters) - 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 election prospects. 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.  More than half a million Britons signed a petition to bar him from entering Britain, where he has business interests, in response to his call to temporarily ban Muslims from entering the United States. British lawmakers decided against a ban as a violation of free speech. 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 in the run-up to the Nov. 8 election to succeed Democratic President Barack Obama. Trump’s swipes at rival candidates and heated exchanges with others have bolstered his standing in nominating contests and opinion polls. One of Trump’s Republican rivals, former Florida Governor 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.” 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’ ultimate trophy, I can promise you that the pope would have only wished and prayed that Donald Trump would have been president.” ISIS is one of the acronyms used for Islamic State.  Trump was in South Carolina, which on Saturday will hold a Republican nominating contest. At a later town hall meeting televised on CNN, Trump said he had “a lot of respect” for Francis but that the pope had been influenced by hearing only Mexico’s side of the border issue. The pope’s statement also had been exaggerated by the media, he said. “I think he said something much softer than it was originally reported by the media. I think that he heard one side of the story, which is probably by the Mexican government,” he said. “He didn’t see the tremendous strain that the border is causing us with respect to illegal immigration, with the drugs pouring across the border.” Thomas Groome, director of the Boston College Center on the Church in the 21st Century, said Francis’ comments were entirely in keeping with his focus on mercy. “The pope is commissioned to preach the Gospel of Jesus Christ. That’s his job,” Groome said. “So when he was asked a direct question, he gave Trump the benefit of the doubt, he said we have to be sure he said this, but if he said this, it is not Christian.” Groome called Trump’s suggestion that Islamic State militants would target the Vatican egregious. “Now it becomes a challenge to ISIS,” he said. Patrick Hornbeck, chairman of the department of theology at Fordham University in New York, said Francis’ words were not surprising given the poverty he had just seen in Mexico. “There is very little common ground between Pope Francis and Donald Trump,” Hornbeck said. He predicted the pope’s words on electoral politics would have little effect on any U.S. Catholics who liked Trump as a candidate. Trump has said he would deport millions of illegal immigrants if he wins the White House. Last week, responding to the pope’s plan to visit the U.S.-Mexican border, he said Pope Francis did not understand the issues. “The pope is a very political person ... I don’t think he understands the danger of the open border that we have with Mexico,” Trump told the Fox Business Network. Asked about being called a “political person,” Francis said on Thursday: “Thank God he said I was a politician because Aristotle defined the human person as ‘animal politicus.’ So at least I am a human person.” Republican Catholics appear to support Trump more than other Republicans do, according to a Reuters/Ipsos opinion poll that showed 43 percent of likely Republican Catholic voters supported Trump, compared with 38 percent of Republican voters generally. The Pew Research Center has said 71 percent of the U.S. population identifies as Christian. That includes the 21 percent of the U.S. population that identifies as Catholic.  The pope was winning the social media battle on Thursday, with overall sentiment negative for Trump and positive for Francis, according to social media analytic firm Zoomph. Author Dan Dicker @Dan_Dicker tweeted: “Let’s see @realDonaldTrump insult his way out of this.” Trump’s social media director, Dan Scavino @DanScavino tweeted: “Amazing comments from the Pope - considering Vatican City is 100 percent surrounded by massive walls.” Evangelical Christian leader Jerry Falwell Jr., who has endorsed Trump, described him as generous to his employees and family, adding: “I’m convinced he’s a Christian. I believe he has faith in Jesus Christ.” Trump was not always at odds with the pope. In 2013, the year Francis began his papacy, Trump compared himself to the pope favorably. “The new Pope is a humble man, very much like me, which probably explains why I like him so much!” Trump tweeted on Christmas Day 2013.  (Reporting by Emily Flitter and Steve Holland in South Carolina and Philip Pullella aboard the papal plane; Additional reporting by Anjali Athavaley, Susan Heavey, Chris Kahn, Scott Malone, Emily Stephenson, Amy Tennery, Mohammad Zargham and John Whitesides; Writing by Howard Goller and John Whitesides; Editing by Toni Reinhold and Peter Cooney) This article was funded in part by SAP. It was independently created by the Reuters editorial staff. SAP had no editorial involvement in its creation or production.