The only study on the performance on Congressional stock portfolios is from 2004 [1]. Do you know of any post-STOCK studies?
[1]: Ziobrowski, A.J., Cheng, P.X., Boyd, J.W., and Ziobrowski, B.J. (2004) “Abnormal Returns from the Common Stock Investments of Members of the United States Senate.” Journal of Financial and Quantitative Analysis, Vol. 39, No. 4, pp. 661-676. (http://www.walkerd.people.cofc.edu/400/Sobel/P-04.%20Ziobrow...)