On paper, the most important speech of the Republican convention will be delivered by Mitt Romney on Thursday, as he accepts the party's presidential nomination. But the one that could make the most difference of all comes tonight. And it won't be delivered by a politician – at least not a professional politician.

Under the revised, Isaac-compressed schedule of the convention, Ann Romney will have some prime-time competition in the hefty shape of New Jersey's combative and trenchant Governor Chris Christie (though not, it would seem, Donald Trump).

But Mr Christie is just a politician. Ms Romney has the crucial task that could conceivably decide the election – of making Americans (particularly American women who currently prefer Barack Obama by a wide margin) to like and trust her husband enough to send him to the White House.

The simple fact is that Ms Romney is a far better campaigner than her husband. All too often he comes across as stiff and wooden. She, by contrast, is the perfect candidate's wife. Where her husband can appear unearthly, robotic and managerial, untouched by the disasters that plague the lives of ordinary people, Ms Romney comes across as straightforward and natural. In a word, normal.

At 63, she has the qualities of the traditional Republican First Lady – not a career woman like Hillary Clinton or Michelle Obama, much more the supportive wife and homemaker in the mode of the two Bush presidential wives, Barbara and Laura. The family is suitably picture perfect too, a handsome husband and five strapping and wholesome sons

But if Ms Romney wears the coat of wealth and privilege, right down to a passion for equestrianism and ownership of Rafalca, a dressage horse that competed in this summer's London Olympics (it finished 28th), the reality is more complicated. Ms Romney has weathered her share of disasters. She has had two reported miscarriages, she has overcome breast cancer. She suffers from the incurable and potentially debilitating illness of multiple sclerosis.

All this makes her the ideal "humaniser" of her husband. Some have dubbed her the "Mitt-igator". Mr Romney needs help in that vital presidential-preference category, the candidate "you'd most like to have a beer with". As a teetotalling Mormon, of course, he doesn't touch the stuff. But if anyone can help square that circle, it's Ann.

Contrary to appearances, she is a first-generation American, whose father, Edward Davies, emigrated from South Wales to the US in 1929. He became a successful businessman and settled in the wealthy Detroit suburb of Bloomfield Hills. Ms Romney met her future husband in 1965 when she was just 16. The following year she converted to Mormonism and in 1969 they were married. Five children followed.

Nothing, however, prepared her for the day in 1998 when she was diagnosed with MS. "I was pretty desperate, pretty frightened and very, very sick," she told the Associated Press in 2004. But she has managed keep the symptoms under control with a variety of treatments that include acupuncture and horse riding.

Tonight though, Ms Romney will be selling her husband to the American people. The Mitt Romney she'll be describing is not the bean-counter at Bain Capital, the competent Governor of Massachusetts or even the rescuer of the 2002 Winter Olympics at Salt Lake City, but the guy who stood by her when the doctors told her she had MS.