Here are some excerpts from an article that J.H. Snider, President of iSolon.org and a fellow at the Edmond J. Safra Center for Ethics at Harvard University, wrote for Politico:
Last week’s appointment of Sen. Jim DeMint (R-S.C.) to head the Heritage  Foundation illustrates an important evolutionary change, going on for more than  forty years, in Washington’s think tank community. Although there are many  exceptions among Washington’s hundreds of think tanks, think tanks as a rule  have been evolving from a type of organization that pursues disinterested public  policy research, “a university without teaching,” to one that pursues  research-based advocacy, “a lobbying shop with research staff.” Launched in  1973, Heritage has been a leader in this evolution. 
The change in think tank leadership reflects a larger change in think tank  personnel. The proportion of full-time think tank staff with Ph.D.’s, let alone  former tenure track academics, has long been in decline.
DeMint’s move to Heritage also illustrates several other important think tank  developments. That one of America’s most powerful politicians is willing, for  the first time in U.S. history, to give up a seat in the Senate to lead a think  tank as a full-time job, is testimony to the growing influence and wealth of  modern think tanks.
The degree of taxpayer support for think tank compensation is also striking.  Since think tanks are funded via tax deductible charitable donations, DeMint’s  compensation will continue to be largely paid for, albeit indirectly, by  taxpayers. Given the large step up from his former $174,000 salary as a member  of the Senate, he may now cost U.S. taxpayers more as a think tank president  than a U.S. Senator.