The eye-bulging “purple fits” of Bill Clinton over recent weeks have seen him taking a reporter to task for being “accusatory with me”, raging about the “fairy tale” of Barack Obama’s position on Iraq and losing his temper in a television interview as aides scurried to stop the recording.

Hillary Clinton supporters worry aloud about him becoming a distraction from her own message, but others say that the “constant anger” inside him serves a purpose by drawing attention to any issue in the way that only such a towering political presence can. As a former president his role in his wife’s campaign is unique but the part being played by many other spouses has evolved from being merely campaign props to that of protagonist.

Michelle Obama has become a formidable figure on the trail. She speaks out of turn occasionally but is fluent enough to vary her laugh lines in stump speeches – a feat that few politicians attempt. She works hard to win over female voters by describing her family life with two young daughters, which she says makes them a lot more normal than most candidates.

Her greatest value this week may be in addressing the black voters that will account for half the turnout in the South Carolina primary. She has family in the state and a more typical African-American experience than Mr Obama’s exotic multinational background.

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

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All the leading Democratic candidates are lawyers married to lawyers, but Elizabeth Edwards has been more effective than her husband, John, in transcending a profession that is often the butt of jokes in America. While her husband sometimes comes across as a clever courtroom advocate Mrs Edwards is almost universally admired. She has written a bestselling book, Saving Graces, in which she discusses the death of her son and a battle with the breast cancer that will probably kill her. Her status has allowed her at times to take an aggressive stance, particularly towards Mrs Clinton, who she claims would do less for women than Mr Edwards.

The wives of Republican candidates generally have assumed a more demure role in this campaign, but John McCain’s second wife, Cindy – a wealthy philanthropist and former rodeo queen – often introduces him at rallies. With her flowing blonde hair, pancake make-up and short, leather jackets, she cuts a striking figure. At his South Carolina victory party on Saturday she appeared to have had a First Lady makeover, turning up in an elegant purple suit with her hair pinned up.

Mitt Romney’s wife, Ann, was a stay-at-home mother of their five boys and has fought multiple sclerosis. She makes the granola that he eats every morning and has featured heavily in his TV adverts and campaign rallies – perhaps to emphasise that unlike other Republicans, this Mormon is still on his first marriage.

Judith Nathan, the third wife of Rudy Giuliani, has kept a lower profile than her initial appearance in which she allowed herself to be photographed kissing her husband passionately. There has been some embarrassment over the soaring cost of his security as Mayor of New York when they were having their premarital affair.

Chelsea Clinton is doing her best to soften the brittle image of her mother, at whose side she has been a loyal, if silent, presence for the past month.

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