Here is a rare and wonderful thing: an analysis of how DC think tanks really operate:
The problem with too many think tanks is that they have become captive of politics. Many people in these organizations aspire to be political appointees—indeed, many are former political appointees. So they say and write what they believe will help them land a job in the current administration or what they hope will be the next administration. Too often that means pulling their punches, which only perpetuates the status quo thinking that permeates Washington. The think tanks tolerate this because their donor bases are increasingly political, seeking to use what should be apolitical institutions to advance their political agendas.
I might add that the expectation that think tanks are independent of political parties is, so far as I can tell, a belief common only in the US. In Europe and the UK, it is taken as a given that the purpose of the think tank is to serve as an idea house for a political faction and visa versa.