Candidates’ spouses have been an important campaign presence since 1920, when Florence Harding spoke to women’s groups from her front porch.

AP reports that this year’s presidential spouses provide a study in contrasts on the trail, in ways that go beyond the personal style differences of these two women — Michelle Obama, a 44-year-old law-school graduate, hospital executive and mother of two young girls, and Cindy McCain, a 54-year-old businesswoman, heiress, philanthropist and mother of four older children.

The traditional, unwritten rule of political campaigns is that you let the candidates sling the mud, while the spouses cheer them on.

But Cindy McCain this month introduced her husband at a Pennsylvania rally with a reference to Obama and his vote against a bill funding the troops in Iraq.

"The day that Sen. Obama cast a vote to not fund my son when he was serving sent a cold chill through my body, let me tell you," the usually quiet, conservative Cindy McCain told the crowd.

Earlier this month, when Democrats were accusing the Republicans of dirty campaigning, she turned the tables and told reporters that Obama ran "the dirtiest campaign in American history," according to The Tennessean newspaper.

She usually can be seen standing behind her husband as he speaks, laughing, applauding, nodding her head in agreement – always a tireless presence on the trail. She doesn’t shy away from telling people her views, but she doesn’t seek it out, either.

Michelle Obama campaigns on her own and rarely is seen with her husband on the trail. She’s gone on talk shows like Jon Stewart’s "DailyShow," "Larry King Live,” "The View” and “Ellen.”

She used to describe Barack’s morning breath or failure to put away the butter while on the trail, but now she focuses on parallels between her life and the concerns facing ordinary Americans and, particularly, military families. She also balances life at home with two young children.

In other news, AP reports that a new study says women running for top offices need to appear competent and attractive, but for male candidates, looks don’t matter much.

"What we found was quite startling," said Joan Y. Chiao of Northwestern University’s psychology department.

"For female candidates, it really matters if they’re perceived as competent and perceived as attractive. Those two qualities are sort of twin predictors of whether or not someone is going to be more or less likely to vote for them."