Why is John McCain doing an ad with his mother? The most obvious reason why his 96 year-old mom will be trotted out from time to time is to combat the age issue. Mental clarity and physical vigor don’t fade fast in the McCain family–that’s the underlying message.

But another reason for the ad is to try to lighten his image. That’s a common way campaigns use wives and other relatives. The ever-teary George H.W. Bush and Laura Bush helped convince voters that George W. wasn’t one of those hard-hearted Republicans. Likewise, the poised Cindy and McCain’s mom are there to convince voters that there is a softer and funnier side to McCain. And that’s in large part why McCain is a regular on Letterman and Daily Show.

Do these efforts work? Perhaps at the margins, but the real impact that close relatives (yes, Bill and Michelle, we mean you) can have is a potential negative one: the gaffes they make reinforce negative images about the candidate. If they can actually help solve a problem, that’s just gravy.