The BLS produces an annual report on the working poor.
The working poor are defined as persons who, during the year, spent 27 weeks or more in the labor force (working or looking for work), but whose incomes still fell below the official poverty level.
Frequency and Timespan: Annual with reports from 1996 to the present
Geographic Level of Coverage: National
Publications and tables available online
United States. Bureau of Labor Statistics, A Profile of the Working Poor Accessed 7/10/2014