The "middle class" is an amorphous concept, and the presidential candidates are giving it their own definitions to fit their political purposes. Here are some ways to define it, political, economic and otherwise:

POLITICS: In pushing for tax benefits for "middle class" Americans, President Barack Obama defines them as families making less than $250,000 — which is 98 percent of U.S. households. He has also described them as having aspirations of owning a home, having affordable health care and being able to pay for their children's college educations. Republican challenger Mitt Romney has defined middle class as families making less than $200,000. He casts them as hard-working Americans who have been "kicked in the gut" in the weak economy.

WHITE HOUSE COUNCIL OF ECONOMIC ADVISERS: In a January speech noting the middle class has shrunk to 42 percent of households, Alan Krueger, who chairs the council, defined them as making annual earnings within 50 percent of the U.S. median income. The current median income is $49,445, putting middle-class earnings in a range from about $25,000 to $75,000.