We have focused in the issue in the 2-nd Chapter of our book. We sent the reader to the Lichter – Rothman study (Lichter S.R. The Vanishing Conservative — Is There a Glass Ceiling?) see http://www.thedivineconspiracy.org/Z5238L.pdf
or in paperback: The Politically Correct University Problems, Scope, and Reforms / Ed. By Fr.M. Hess, R. Maranto, R.E. Redding. AEI Press (September 2009).
BTW NYT presents own coverage of the same issue.
The article references is the Fosse, Gross et al studies on the Graduate admission process and Political liberalism and Graduate school attendance .
In the first paper authors denied significant dependency of the application process and mentioned activities of 2008 Obama (McCain) campaign.
In the latter paper authors found liberal shift explained it by conservative graduates’ less desire to join and activities dominated by liberals, to start the academic career as the Academy already overpopulated by the leftists.