Mark Sanford is not the first Republican to turn against former US president Donald Trump and pay a political price. But he is unique in publishing a memoir that links his decision to lessons learned from an extramarital affair and his attempt to cover it up.

Sanford was the governor of South Carolina when, in 2009, he flew to Argentina to be with a woman who was not his wife but told his staff he was hiking the Appalachian Trail. The falsehood was rapidly exposed, made lurid headlines and ended his 20-year marriage.

The story is retold in frank detail in Sanford’s book, Two Roads Diverged, published on 24 August, which charts how the Christian conservative’s personal and political journey led him to follow his conscience and make a stand against his beloved Republican party’s moral capitulation to Trump.

“I just figured, if you want to have a conversation about trust and where the party goes from here, I needed to come clean and say, hey, I’m not trying to hide something,” the 61-year-old explains by phone from home in Mount Pleasant, South Carolina. “We all got stuff but here’s my stuff. Here’s how it went down. Here’s what happened. But here’s why it gave me an acute appreciation for the truth.”

It was in December 2008, Sanford writes, that his wife Jenny discovered a file of correspondence between him and María Belén Chapur, an Argentinian journalist he had met seven years earlier. This led to marriage counseling and, in May 2009, a request from Jenny for a trial separation. “One month later,” Sanford writes, “I chose a path that caused incalculable pain for all of us.”

On 18 June he flew to Buenos Aires for what he admits was “the strangest of plans” to find a resolution. He reckoned that if he could sample Belén’s “country club lifestyle” in Buenos Aires, which was at odds with his own, then he “would fall out of love and get my life back”.

To paraphrase TV host Jay Leno, what the hell was he thinking? “ I fell in love,” Sanford says with disarming honesty. “Deeply in love. I don’t know that we make all the most rational decisions when we’re head over heels in love.”

On leaving his office for the weekend, Sanford had told one staff member that, if Jenny called, they should say he was “hiking on the Appalachian Trail”. But within hours a political adversary was demanding to know his whereabouts.

Speculation about the governor’s mysterious disappearance intensified. Eventually, at 10.05pm on Monday, a member of his staff issued a statement that Sanford says will haunt him to his grave: “Governor Sanford is hiking the Appalachian Trail.”

Governor Sanford was in fact on the Pampas, the wide open grasslands of Argentina, with Belén. When his office called him later that night, he knew the game was up.

“You know your world has ended, you just don’t know what it’s going to look like,” he recalls. “You’re going to your execution, you just don’t know its form. It’s a wrenching loneliness and sense of despair. How did I get here? What do I do? What can I do? Nothing. Go back and reap the consequences of where you are.”

Sanford returned home and gave a rambling press conference, apologising and confessing: “I’ve been unfaithful to my wife … with a dear, dear friend from Argentina.” His lie about the Appalachian Trail became a punchline on late-night TV shows. He faced calls to resign and five different investigations; articles of impeachment were eventually dropped but he was censured.

Jenny filed for divorce; the most difficult conversation of all was with their four sons. Sanford and Belén got engaged only to break it off two years later. The entire episode led him to reassess his life and values and seek redemption.

“It completely changed my view toward judgment,” Sanford reflects. “Previously in my life, you’d read the newspaper and you’re thinking, idiot, moron, what was he thinking or what was she thinking? Now it’s like, there but for the grace of God go I.”

Despite everything, Sanford completed his term in 2011 – he was succeeded by Nikki Haley – and returned to politics two years later, winning a seat in the House of Representatives vacated by Tim Scott, who had moved to the Senate. He was twice re-elected but in 2018, he writes, his second chance “came to a screeching halt”.

The reason: Trump.

“In a cosmic sense, it’s like, God, is this just a cruel joke? I went through a rather humbling journey post-2009 wherein, needless to say, within the GOP there were some folks who felt strongly that I’d let them down, which I had …

“And then fast forward and along comes a guy who personifies all of the opposite things that I thought God and circumstance and other people were trying to teach me post-09. And this guy is running for president? And then he gets the nomination and then he becomes president.”

Sanford could never be described as a liberal hero. He was a hardline conservative and fiscal hawk. The Confederate flag flew in the South Carolina statehouse grounds when he was governor and he still opposes the removal of Confederate statues (“the notion of cleansing history seems to me a huge mistake”).

Even so, he watched in horror as Trump seized the Republican party in 2016 and politician after politician sacrificed their principles and bowed before this bully who gave the perception of strength. “It’s at the end of the day about what politics is always about, which is power.”

Can he remember what he found so objectionable? “Everything,” he says simply. “It was the lack of humility, the lack of embrace of of conservative ideals that I thought mattered, the lack of a Socratic approach, at least a belief in ideas themselves whether from the left or right, the lack of belief in the institutions that our founding fathers put in place. The idea that the media could be an enemy of the state.

“The whole thing was all so contrary to the idea of a pluralistic society and an institution of checks and balances. It’s like this guy would make a great dictator, but that’s not what the American system is about.”

He adds: “Mind you, I wasn’t alone at that time. The Freedom Caucus, of which I was a part, basically went all out against him initially in what they said but then they all flipped, except for people like Justin Amash who I have great respect for.”

Amash has now left the House. Bob Corker and Jeff Flake have left the Senate. Liz Cheney has lost her position in House leadership. What they have in common is that they are NeverTrumpers in the Republican party, heretics against the dominant religion. Sanford met the same fate.

“It got lonelier and lonelier fast,” he says. “I remember having a rather heated conversation with a major donor of mine and his point was, the guy is far less than perfect but he is a Republican president and there’s only one quarterback and it’s your role to support the quarterback. My point was, no, it’s my role to support the ideas and ideals that the party has stood for, that the quarterback is actually supposed to be supporting too.”

Three years ago Sanford was beaten in a Republican primary election by a Trump-backed candidate (the president tweeted: “He is better off in Argentina”). It was the kind of result that strikes “terror”, as he puts it, into other congressional Republicans and helps keep them in line. But he has no regrets.

“It was a big spiritual night. The four sons were with me and afterward we went over to this place called Cook Out, a cheap burger place. We stayed there until three in the morning talking about life and politics and their investment in it: that’s all they’d ever known, everything they’d grown up with.

“To a boy – or young man now – they all said, Dad, if you had to go out, this is absolutely the way to go out.”

So who did Sanford vote for in the 2016 and 2020 presidential elections? He prefers to keep his answers off the record, except to note that it easy to guess who he did not vote for. Trump’s four-year presidency lived down to his expectations, culminating in the “otherworldly” insurrection at the US Capitol on 6 January, and proved such a turn-off for suburban women that he fears they could be lost to the Republican party for a generation.

Seven months later, does he see signs that Trump’s grip is loosening, that the fever might break? He points to the example of his friend, fellow South Carolinian Lindsey Graham, a senator who at first denounced Trump but is now his regular golf buddy. “We started in Congress together and he’s very much of a different school on these kinds of things and adapts where he needs to adapt to hold power.

“But I would use him as a canary in the coalmine and the degree to which he has doubled, tripled and quadrupled down on Trump says everything. Whether you like him or not, he has a good political nose for his base. He knows South Carolina well and his reading the tea leaves is not to be dismissed.”

Sanford hopes to remarry some day – “I don’t want to face life alone” – and, after an abortive presidential primary campaign against Trump in 2019, thinks another political comeback is unlikely, although “if there’s ever a guy who would never say never, it would be me”. For now, like other anti-Trump Republicans, he finds himself wandering the wilderness.

“I am a citizen with no country. I am an unabashed conservative in the Jeffersonian sense and right now, if you believe in those ideals, you certainly don’t find it in the Republican party and it’s even been lost within the movement itself of late because there is no movement.”