Ann Romney has a story about the day Mitt told her that nothing he had ever done matched her achievement in raising their five boys.

She liked hearing it, she says simply, before quipping that when he becomes president she won't be able to compete any longer.

The drudgery of being a wife on the primary circuit cannot be overstated - all the smiling at speeches and jokes as if hearing them for the first time.

Karen Santorum was against her husband running until she realised it was "God's will". Ann Romney was happy at first but has recently shown signs of impatience, particularly with the press. Only Carol Paul, wife of Ron Paul, is spared. She remains mostly at their Texas home.

Spouses are always more than ornaments to voters who are hungry to know them. They can quickly become an asset or a drag, or sometimes both. Even as First Lady, Michelle Obama still draws conflicting reviews. Does her strong character and competence make her an indispensable partner to the President or a political liability?

In the current Republican primary season it was Team Romney that first decided Ann was going to help. The Santorum and Newt Gingrich campaigns were more wary of rolling out their respective other halves.

Callista Gingrich, who turns 46 today, may have seemed especially problematic. She is the former House speaker's third wife and was his mistress for six years. And her helmet coiffure fascinates; they say she could ride on the wing of Air Force One at 40,000ft and return to earth without a hair out of place.

Wives matter too because of the female vote: more women vote in the primaries than men. In this regard Romney, who also frequently takes his sons on the road, each of whom could model for a preppy clothing brand, does fine. In Michigan last week, he beat Santorum among women by five percentage points.

This statistic has not escaped advisers to Santorum who devoted much of his election night speech to the women in his life, including his mother and wife, who he said is "as strong as they get," noting that she quit her law practice to bring up their children.

For consolidating support among Christians, Karen Santorum is the perfect partner. When not on the road, she cares for their youngest child, Bella, who has a genetic defect. But other details of their devotion might put some voters off.

When they lost their child Gabriele, soon after birth, they slept with the dead infant between them in the hospital bed and then took him home so their other children could see him before releasing him to the morgue.

Karen Santorum used an interview with conservative broadcaster Glenn Beck last month to explain how she overcame her initial reticence to his running.

"God has big plans" for Rick, she revealed, adding that He "has us on a path. I do think that there's a lot more happening than what we're seeing".

Gingrich is never to be found out on the trail without Callista. She, too, was not keen on his running and allegedly threatened to dump him last summer when he considered cancelling a Mediterranean cruise to instead campaign.

It did not help when it surfaced subsequently that Newt had a US$500,000 ($600,000) line of credit with Tiffany's to keep her in pearls. Only in the past few weeks has Callista turned from mannequin to live human.

Aware that the candidate was polling disastrously among women, the Gingrich campaign finally decided her to show off her ability to speak, beginning by introducing Newt at the CPAC conservative conference in Washington.

Born with Welsh roots - her grandfather, David Davies, was a coal miner - Ann Romney cuts an attractive figure as well as a courageous one. Sometimes, Romney mentions her challenges: she underwent a breast lumpectomy in 2008 and was diagnosed with multiple sclerosis in 1998, although the disease has not prevented her leading a normal life.

Recently there have been signs of testiness, however. Signalling she has had enough of the TV debates, she said that if there are any more she would do them, not Romney. And last week she told one reporter she felt like "strangling" the press and wondered about keeping certain reporters off the Romney bus.

It is a feistiness glimpsed on primary night in Michigan. Introducing Romney, she told everyone to shut up while she went through the thank yous.

"I am going to see if you are all going to behave and listen to this list without cheering in between - let's see if you can get this right," she said. Yes, matron.

- Independent