What is meant by "The Middle Class"

A couple weeks back, the NPR Ombudsman wrote in her blog about what is meant by the term "Middle Class."   This word's an important one, as it's getting thrown around a lot in current political discourse, especially when people talk about whether a certain policy will "help" or "harm" the middle class.

As the Ombudsman and a Senior Editor at NPR point out, there's really no set definition of the middle class, other than the "average" of what most Americans make (around 42k yearly income).  In short, however, the use of the world "middle class" seems to be an easy way for politicians to appeal to the greatest amount of Americans possible while still sounding like they know what they're talking about.

What I found most interesting, though, was that in Britain politicians might also talk about the "upper class" and the "lower" or "working" class.

In journalism, it seems like the distinction between working class and middle class would be an important one.  Is a janitor, or carpenter, or truck driver who makes just above 30k per year described as "middle class" when a lawyer or tenured professor making up to 90k per year might also be considered "middle class."

That might not be a distinction that politicians in Washington are willing to make-- they'd be painted as Marxists or snooty nobles if they did-- but it's an important for journalists who are supposed to be telling accurate stories.  For example: are middle class voters the same as lower class voters? Upper class voters? Do they vote the same way?  Seems to me that we're missing out a big part of the story by confusing the two.