US President Barack Obama has defended Ann Romney, the wife of his main Republican rival, saying there is "no tougher job" than being a mother.

He spoke out amid an political row that flared after a Democratic strategist criticised Mrs Romney, saying she had "never worked a day in her life".

Politicians were "fair game", but their families may not have "volunteered" for the job, Mr Obama said.

Mrs Romney, 62, raised five children and was diagnosed with MS in 1998.

"I don't have a lot of patience for commentary about the spouses of political candidates," Mr Obama said, speaking to local news channel KCRG.

"My general view is those of us who are in public life, we're fair game. Our families are civilians. I haven't met Mrs Romney, but... I don't know if she necessarily volunteered for his job."

Earlier on Thursday Hillary Rosen apologised for the original comment, saying: "As a mom I know that raising children is the hardest job there is. As a pundit, I know my words on CNN last night were poorly chosen."

"I apologise to Ann Romney and anyone else who was offended."

'Hard work'

In response to Ms Rosen's initial remarks on Wednesday, Mrs Romney set up her own Twitter account.

"I made a choice to stay home and raise five boys," her first tweet read . "Believe me, it was hard work."

Image caption Ann Romney is a frequent presence at her husband's side and introduces him at events

Then First Lady Michelle Obama then entered the fray, tweeting on Thursday : "Every mother works hard, and every woman deserves to be respected."

Senior Obama campaign advisers David Axelrod and Jim Messina also posted tweets on Wednesday in support of Mrs Romney.

The exchange comes as the Obama and Romney campaigns vie for women's votes ahead of November's presidential election.

A recent poll found that among women voters, President Obama led Mr Romney 54-36%.

During Wednesday's CNN appearance, Ms Rosen argued that Mr Romney was using his wife as a surrogate on women and the economy, and that Mrs Romney had not faced having to raise children and work.

Ms Rosen, who is mother to twins with her former partner Elizabeth Birch, accused critics of scoring political points off her comment.

"They're attacking me. That's fine, but it does not erase [Mr Romney's] woeful record," she said on CNN on Thursday.

"Spare me the faux anger," Ms Rosen wrote in a longer response on Huffington Post, saying she has "nothing against Ann Romney".

"She seems like a nice lady who has raised nice boys, struggled with illness, and handles its long-term effects with grace and dignity."

In an interview with Fox News, Mrs Romney took exception to the characterisation of her husband as being out of touch with women and said she was hearing similar themes from women on the campaign trail.

"I don't care if they are stay-at-home moms, or they are working mothers, or grandmothers. Guess what they are all talking about?" Mrs Romney said.

"They are talking about jobs, and they are talking about the legacy of debt that we are leaving our children."