A summary of the work, The Missing Class by Katherine Newman and Victor Chen on at the New America Foundation.
They are neither the middle class, nor the traditional blue-collar (now shrinking) unionized working class nor the poor. They are the near poor. They also focus on those who moved up to the near poor status from poor via Welfare Reform and the late 90s economy. Rather than those who have fallen into the near poor status via home foreclosure, health care costs rising, loss of jobs.
Also an interview with Newman in the Nation here.