This isn't to say internal politics aren't involved in tenure decisions, but actual politics?
John Hasnas, writing in the WSJ:
"In the more than 20 years that I have been a professor at Georgetown University, I have been involved in many faculty searches. Every one begins with a strong exhortation from the administration to recruit more women and minority professors. We are explicitly reminded that every search is a diversity search. Administrators require submission of a plan to vigorously recruit applications from women and minority candidates.
Before we even begin our selection process, we must receive approval from the provost that our outreach efforts have been vigorous enough. The deans and deputy deans of each school reinforce the message that no expense should be spared to increase the genetic diversity of our faculty.
Yet, in my experience, no search committee has ever been instructed to increase political or ideological diversity. On the contrary, I have been involved in searches in which the chairman of the selection committee stated that no libertarian candidates would be considered. Or the description of the position was changed when the best résumés appeared to be coming from applicants with right-of-center viewpoints. Or in which candidates were dismissed because of their association with conservative or libertarian institutions."