In American presidential campaigns, September is the cruellest month. It is the time, following the long summer break, when partisan warfare resumes in earnest. It is the moment when the gloves really come off. The primary season is past, the Democratic and Republican party conventions are over, the presidential nominees have been formally acclaimed. Now begins the hard, fast, unforgiving head-to-head run towards November’s national winning post.

Hillary Clinton should be cantering towards a clear victory. After 25 years in public life, the Democratic candidate has unmatched experience. She has been tested in roles as varied as first lady, senator and secretary of state. She has raised the profile of women in politics to new heights, taking her lifelong role as “ceiling-cracker” (her word) to the doors of the Oval Office. In Donald Trump, she faces a fourth-rate rival who has proved himself unfit for office, a bigoted, ignorant man who feeds off lies and hate. She should be a shoo-in.

Instead, September has seen Clinton stumble. As independent and undecided voters begin to focus on their choice, Clinton’s unusually poor approval ratings are assuming critical importance. Trump’s risible, insincere attempts to move away from the extreme right have given him an undeserved bounce. Clinton’s averaged-out poll lead is down to less than two points. In some surveys, Trump is actually moving ahead.

Clinton’s biggest stumble was a physical one, in New York last Sunday when she fainted, almost fell, lost a shoe and had to be helped away. The subsequent furore, her belated admission she was suffering from pneumonia, the realisation this had been concealed for days and the suspicion she was hiding something worse conspired to highlight one of Clinton’s two big vulnerabilities: lack of trust. Put bluntly, when she states a fact, makes a pledge or offers an explanation, there is a widespread tendency not to believe her.

Trust is always a big issue in presidential campaigns. But in Clinton’s case, as has become customary throughout her career, she is constantly held to a higher standard than her contemporaries. She has, for example, been relentlessly castigated over her use of private email when secretary of state. Ideally, she would have adopted a more secure method of communication. But not even the most rabid Republicans have demonstrated any serious consequences as a result. That predecessors did the same thing and escaped censure is not viewed as a double standard.

Clinton was in charge when the US mission in Benghazi, Libya, was attacked and its diplomats killed in 2012. But to blame her, rather than Barack Obama, or the CIA, or the Marine Corps – whose job is to protect embassies – is grossly unfair. To criticise Clinton for her refusal to divorce Bill after his relationship with White House intern Monica Lewinsky was revealed is to treat her very differently from the partners of other erring politicians.

Clinton is still dogged by the so-called White House travel office scandal of early 1993, an insignificant affair but one that established the subsequent, long-lasting narrative of a bossy, arrogant, untruthful person who brooks no criticism. Whether the issue is healthcare reform, which she attempted as first lady and was pilloried for, or sensitive questions of equal rights, which she has championed with growing confidence in recent years, Clinton has been hounded at every turn in a hostile, frequently crude manner that other public figures simply do not experience.

A common factor in all of these controversies is the fact that Clinton is a woman – which despite all that has been achieved over women’s rights, remains her other big political vulnerability. For being a Yale-trained lawyer and partner in a leading law firm, for retaining her maiden name of Rodham, for insisting on maintaining an independentlife while married to a president, for pursuing a political career in tandem with motherhood, and now for presuming to become America’s first female leader, it seems a significant chunk of the public and the media cannot forgive her.

Clinton has been called a feminazi, a shrew, a scold, a harridan, even a dominatrix who has emasculated America. Her famous 1992 defence of her troubled marriage – “I’m not sitting here some little woman standing by my man like Tammy Wynette. I’m sitting here because I love him and I respect him” – was misrepresented as proof that she looked down on traditional female roles. She fuelled the misogynist fire by defending her legal career: “I suppose I could have stayed home and baked cookies and had teas but what I decided to do was fulfil my profession, which I entered before my husband was in public life.”

For supposedly outraged conservatives, the storyline of Hillary, untrustworthy betrayer of American womanhood, was set in stone. It has not changed much since, and according to Tammy Vigil of Boston University, Clinton is still struggling with it. “Right out of the gate she was getting slammed for the pantsuits, the hair, the headbands, her appearance, her life choices, and everything she said was so heavily scrutinised. I think she became defensive. And now she’s trying not to be as defensive, but it’s sort of still there.” Even her recent illness was used to fortify this critique: namely, that Clinton, unlike the 17-stone, 70-year-old Trump, cannot take the strain, cannot be trusted to tell the truth, and is unfit for office,

Far from undermining women, Clinton is a standard bearer on the cusp of an historic breakthrough. It may be said she lacks a new vision of governance, that her ideas are rooted in the discredited Clinton-Blair New Democrat/New Labour era, and that, in foreign affairs, she can be both too aggressive (for example, towards Iran) and too cautious, as on Palestinian rights. In personal dealings, she is undoubtedly sometimes prickly and impatient, though these failings are exaggerated.

At the same time, her achievement in overcoming the extraordinary bias and prejudice ranged against her is admirable. Clinton combines intelligence, courage, energy, determination and a powerful sense of destiny. That she is by any measure the best candidate on offer in November cannot be sensibly denied. That she is America’s last defence against Trump’s appalling sexism, racial prejudice and dangerous xenophobia is seriously sobering. Like her or not – totally trust her or not – Clinton deserves support. Non-voters across the globe must hope she will avoid new stumbles, keeps well – and wins.