Clinton’s chief liability, though, is the baggage she carries of her own. This includes the matter of that private email account she is claimed to have used professionally while secretary of state, and her handling of the murder of the US ambassador in Libya. The latter suggests a reluctance to accept ultimate responsibility, which is not a good recommendation for a president. The former suggests confusion about where to draw the line between the personal and the professional – a line more clearly drawn in US politics than here. Her explanations – most recently to reporters in Iowa, where she talked about “convenience” that turned out “to be not so convenient” – remain unsatisfactory and high-handed.

Most unfair of all is the health question. In her last weeks as secretary of state, Clinton spent a few days in hospital being treated for a blood clot, the prognosis being a complete recovery. When she first mooted running again for the presidency, however, George Bush’s former attack-dog, Karl Rove, cited that incident to sow doubt about her health. He was criticised even by fellow Republicans at the time, but presidential campaigns are ruthless. If she wins the nomination, the health issue will not be off limits.

For Clinton even to consider halting her campaign would, of course, be wrenching, and not only for reasons of personal ambition. As her loyalty to her parents and her wayward husband showed, she is imbued with a deep sense of duty, and duty doubtless plays a role in her campaign. As the one woman with the profile and fundraising capacity to reach the White House in 2016, she would surely prefer to entrust her fate first to the delegates of the Democratic convention and then to the voters of the United States. This would be understandable but misguided. In any pursuit there comes a point of no return, and with a pre-campaign fixture – the Iowa State Fair – out of the way, that point is fast approaching.

As it happens, yesterday was the anniversary of Bill Clinton’s public admission of his relationship with Monica Lewinsky. Today is the anniversary of that gruesome family tableau – Hillary, Chelsea, Bill, and dog Buddy – taking the long walk across the White House lawn to the helicopter that would whisk them away to their holiday on Martha’s Vineyard.

In the 17 years since then, Clinton has become the first former first lady to win a Senate seat, and the first woman to be elected as a US senator for New York. These years also included her defeat for the Democratic nomination to the one candidate in a generation whose appeal outstripped hers. And she spent four of them travelling the world as US secretary of state. It is a distinguished and remarkable career, but it is now time to call it quits, while the decision is still hers to make.

She can cite personal reasons (concerns about her husband’s health, for instance), or the hope that she has left time for another woman – the Massachusetts senator Elizabeth Warren, or the health secretary, Kathleen Sebelius – to run. But call it quits she should, unbeaten.