US election: Donald Trump v Pope... who wins?

Will American Catholics care about Trump's war of words with the Pope?

The one thing we've learnt about Donald Trump since he entered the presidential race is that turning the other cheek is not his style. Ever.

He's lashed out at his critics and bullied his opponents. He has mocked the disabled and had protesters bundled out of his rallies.

Only yesterday he sent a "cease and desist" letter to his Republican rival Ted Cruz, threatening legal action over "lies" he'd allegedly committed. But issuing a writ to the Vatican is not a course of action his lawyers would recommend.

And watching him last night at a town hall event organised by MSNBC he was unapologetic. He says he is merely defending himself against the attacks of others.

But what you do when the person attacking you over your plans to build a wall along the southern US border is not just another "here today, gone tomorrow", two-bit politician - but instead is the pontiff?

We're talking about the leader of the world's 1.2bn Roman Catholics, the Bishop of Rome, the successor to St Peter, the vicar of Jesus Christ, the Holy Father - yes the Pope is all of that and more (good thing he's so well known he doesn't have to produce a business card with all that on it).

The Pope has millions of followers - on Earth and on social media

Well, if you're Donald Trump, you meet fire with fire. The only difference between the manner of his reply to the Pope than to his conventional political opponents was that it was carefully scripted. He didn't ad lib as he normally does. He read very carefully from the words on the page.

It is worth reproducing in full:

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 because this would not have happened. ISIS would have been eradicated unlike what is happening now with our all talk, no action politicians.

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, both on trade and at the border, and they understand I am totally wise to them. The Pope only heard one side of the story - he didn't see the crime, the drug trafficking and the negative economic impact the current policies have on the United States. He doesn't see how Mexican leadership is outsmarting President Obama and our leadership in every aspect of negotiation.

For a religious leader to question a person's faith is disgraceful. I am 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. No leader, especially a religious leader, should have the right to question another man's religion or faith. They are using the Pope as a pawn and they should be ashamed of themselves for doing so, especially when so many lives are involved and when illegal immigration is so rampant.

Not a battle one would have predicted but this has been an unusual election

Mr Trump, who has been a genius in his use of social media, has also been enjoying retweeting one delicious bit of irony - of course, the one city state that has an enormous wall around it is Vatican City in Rome.

But did Mr Trump need to take on the Pope? Well, almost certainly yes. Because in god-fearing South Carolina, the next state to vote in the primary process - to have the Pope say that he is unchristian is potentially very damaging. And over the course of the campaign, the billionaire property developer has been at pains to prove his religious credentials, appearing at rallies with a copy of the Bible that his mother had given him as a child - though, at another, getting the name of a passage of scripture muddled. His detractors had great fun with that.

JFK's faith was viewed with suspicion

If you have grown up in liberal European democracies, where religion will play little or no part in an election, then this focus on faith in the US seems decidedly odd - anachronistic even, in a country where there is a total separation between church and state. Tony Blair's spokesman when he was prime minister, when asked about religion, famously retorted: "We don't do God" (even though Christianity played an important role in the then prime minister's life).

Give that response here in the US and you might as well throw in the towel. Every candidate is asked and has discussed their faith. Democratic contender Bernie Sanders has talked about the importance of Judaism and spirituality in his life, and other candidates have discussed their relationship with Christ and the church. Being an atheist is NOT an option here for a politician seeking high office.

But the Catholic population in the US is nothing like as influential as the Christian evangelicals. Remember in the 1960s, when John F Kennedy was making his run for the presidency? There was a big question mark over whether the US would ever elect an east coast, northern Catholic to be president, particularly in the south.

One feature of Barnum's circus was a so-called freak show

It was for that reason that JFK recognised that he would have little option but to make Texan Lyndon B Johnson his running mate, even though there was little warmth between the two men.

South Carolina is the first Southern state to vote this weekend. And the chances are that the Pope's intervention will do Mr Trump little harm. The state has one of the lowest proportions of Catholic populations of any state in the US. And this is a man who has made a habit of defying political gravity. Things that make ordinary political mortals crash to the ground, just seem to result in Mr Trump rising even higher.

I've always thought it one of the more fatuous epithets that "there is no such thing as bad publicity", often ascribed to Phineas T Barnum, the 19th Century American showman and circus owner.

But in 2016's political circus - where Donald Trump is concerned - it seems bang on.

You can follow Jon @BBCJonSopel

Trump's religious views: In his own words

Donald Trump was raised as a Presbyterian

• "I'm going to protect Christians" (January 2016)

• "I think if I do something wrong, I think, I just try and make it right. I don't bring God into that picture. I don't" ( July 2015)

• "I believe in God. I am Christian. I think The Bible is certainly, it is the book...I'm a Protestant, I'm a Presbyterian. And you know I've had a good relationship with the church over the years. I think religion is a wonderful thing. I think my religion is a wonderful religion." (2011)

• His proposed Muslim ban: "Until we are able to determine and understand this problem and the dangerous threat it poses, our country cannot be the victims of horrendous attacks by people that believe only in Jihad, and have no sense of reason or respect for human life." (December 2015)

• Muslims in general: "Most Muslims are wonderful people, but is there a Muslim problem? Look what's happening. Look what happened right here in my city with the World Trade Center and lots of other places." (2011)