According to an article by James Bovard via the Brownstone Institute, Washington think tanks have played a significant role in shaping federal policy, often to the benefit of those in power. The article suggests that these think tanks, of which there are over 400 in the Washington area, frequently provide intellectual justification for government interventions and expansions of power.
This can be seen as a form of "rent-a-mob" for political protestors, where think tanks offer support for certain policies or ideologies in exchange for funding or other benefits. Some think tanks have been accused of receiving funding from foreign governments with questionable human rights records, such as Qatar and Saudi Arabia, in order to promote a positive image of these countries in the international media.
The article cites a 2014 New York Times report, which found that at least 64 foreign governments or state-controlled entities had contributed to 28 major US-based research organizations since 2011. This has led to concerns that think tanks are being used as a "muscular arm" of foreign governments' lobbying efforts in Washington.
The article also notes that think tanks can serve as a way for politicians to create and spend "political capital," with some think tanks acting as little more than "cash machines for power" for politicians. The president of the Foundation for Defense of Democracies, Clifford May, is quoted as saying that it is the job of think tanks to create political capital, and the job of politicians to spend it.