Rupert Cornwell: All the candidate's women – rules for surviving a scandal

Out of America: Cain may be a goner, but sexual indiscretions do not always lead to political death

Here's a rule-of-thumb for US presidential politics: the strength of a campaign is in inverse proportion to the laughs it provides on late-night television. If that is true, then Herman Cain is a goner, after the TBS channel host Conan O'Brien brought the house down with a spoof campaign ad lampooning the hapless Cain whose alleged sexual adventures have been making news of late.

"Jimmy Carter never cheated on his wife and we had 7 per cent unemployment and 14 per cent inflation," the fake narrator says. "Bill Clinton cheated on his wife every chance he could get and we had only 4 per cent unemployment, 1 per cent inflation and a balanced budget." Then, with suitably imposing muzak playing in the background, the sign-off. "Herman Cain: Because when presidents get laid, you get paid."

And last night Mr Cain duly announced he was "suspending" his campaign for the Republican nomination, following discussions with his wife over the latest allegations he had a 13-year extramarital affair with Ginger White. He vowed to keep fighting, but candidate Cain is no more.

Yet set aside the late night joking. O'Brien raises a fascinating point. Why do some politicians survive sex scandals while others are ruined by them? Tales of sexual misconduct have swirled around presidents and would-be presidents ever since Thomas Jefferson, back in 1802, was accused of an affair with his black slave Sally Hemings. For much of the 20th century, however, a curious truce was observed. Reporters had their suspicions about Warren Harding (who was entertaining young ladies in White House closets seven decades before Bill Clinton met Monica Lewinsky) and later about John F Kennedy. But a clubby, boys-will-be-boys, spirit prevailed.

In the 1970s, though, the rules changed – maybe because of the distrust of cynicism born of Vietnam and Watergate, or perhaps it was the growth of a feminist movement out to ensure men were no longer granted carte blanche in sexual matters.

And so to Gary Hart, the first prominent "victim" of the modern sex scandal. Everyone knew that Hart was "a man who appreciated a well-turned ankle". But by the time he was running for the 1988 Democratic nomination the rumours had become so deafening that the candidate challenged the press: "If anyone wants to put a tail on me, go ahead," he said. "They'd be pretty bored." The Miami Herald took him up on the offer, and was not bored.

Its story turned the aspiring actress Donna Rice, who accompanied the politician on a two-day yacht trip aboard the splendidly named Monkey Business, into a short-lived global celebrity. Hart's campaign was finished. He had committed the two cardinal sins in a political sex scandal. The affair was current and he'd been caught lying about it.

Yet Bill Clinton was to survive even more lurid allegations. That he was prone to "bimbo eruptions" had long been known. But in January 1992, at the height of campaigning for the New Hampshire primary, Gennifer Flowers, a nightclub singer, went public with claims of an affair, with phone tapes to prove it. Like most reporters covering the 1992 election, I assumed Clinton had had it. But that was to reckon without a masterpiece of damage control, as his camp sought to discredit the witness, claiming the tapes had been doctored. It helped, of course, that the press liked Clinton, as they had liked JFK 30 years earlier. Clinton managed to come second in the primary, and labelled himself "The Comeback Kid". The rest is history.

But had he been instantly exposed as a flat-out liar, that surely would have been the end – just as it would be the end, years later, for the presidential campaign of John Edwards after he had denied his affair with the film-maker Rielle Hunter with whom he fathered a child, only to be caught out as his wife was dying of cancer.

And now Cain. Yesterday he again denied the stories ("false and unproven" he called them). But the evidence against him was stacking up, and, if Ginger White is telling the truth, the affair continued even as Cain was campaigning. That is the difference between him and Newt Gingrich, who has supplanted him atop Republican polls. Three times married, Gingrich trails a multitude of personal sins. But they are past, and broadly known about. They may weigh on voters' judgement of him. But they will not force him from the race

JFK was a charismatic politician but he was lucky in the age he lived. Today, he would surely have been washed away by scandal. Ben Bradlee, the former editor of The Washington Post, wrote in his memoirs: "If the American public had learnt he had shared a girlfriend with a top American gangster [Sam Giancana] and Lord knows who else, I am convinced he would have been impeached."

Herman Cain, as they say, is no JFK. But you couldn't blame him for wishing he was running for president 50 years ago.