US News & World Report 
says that employees at all but the most conservative think tanks give more to liberal candidates.
A U.S. News analysis of data from the Center for Responsive 
Politics, a nonpartisan organization that tracks money in politics, 
suggests that employees at all but the most conservative organizations 
gave far more financial support to Democrats than Republicans over
 the last four election cycles. During this time period (2003 through 
2010), Republicans and Democrats each controlled both houses of Congress for four years, and Americans also elected both a Republican and a Democratic president.
Here is their analysis of moderate think tanks:
Employee contributions from some of the top moderate think tanks skew 
decidedly to the left. For example, the Center for Strategic and 
International Studies and the RAND Corporation, two of the policy 
institutes with the most generous employees, have 84 percent and 91 
percent Democratic giving records, respectively. The two think tanks 
with the most bipartisan spread of campaign contributions--the Council 
on Foreign Relations and the Aspen Institute--still have seen more than 
two-thirds of their employees' reported contributions going toward 
Democrats and liberal PACs since 2003. Even employees of the 
Congressional Research Service, sometimes called "Congress' think tank,"
 have given 100 percent of their donations since 2003 to Democratic 
candidates and committees. 
Here is the chart that they compiled:
| Organization | Political Orientation | 2003-2010 Donations | GOP % | Dem % | 
|---|
| Brookings Institution | Liberal | 239,229 | 1.20% | 97.60% | 
| Center for Strategic and International    Studies | Centrist | 169,620 | 12.30% | 83.80% | 
| Center for American Progress | Liberal | 164,227 | 0.30% | 98.80% | 
| RAND Corporation | Centrist | 160,525 | 8.80% | 91.20% | 
| Council on Foreign Relations | Centrist | 146,849 | 30.20% | 69.60% | 
| Hoover Institution | Conservative | 146,830 | 88.40% | 9.60% | 
| Cato Institute | Conservative | 127,800 | 99.60% | 0.00% | 
| American Enterprise Institute | Conservative | 85,495 | 93.00% | 0.60% | 
| Urban Institute | Liberal | 79,259 | 0.00% | 100.00% | 
| Carnegie Endowment for International    Peace | Centrist | 53,175 | 0.90% | 99.10% | 
| Woodrow Wilson Center for International    Scholars | Centrist | 51,895 | 14.50% | 85.50% | 
| Kaiser Family Foundation | Centrist | 49,500 | 0.00% | 98.00% | 
| Center for a New American Security | Centrist | 42,750 | 5.40% | 94.60% | 
| Aspen Institute | Centrist | 35,000 | 21.60% | 78.40% | 
| U.S. Institute of Peace | Centrist | 32,661 | 0.80% | 94.20% | 
| Heritage Foundation | Conservative | 31,646 | 100.00% | 0.00% | 
| New America Foundation | Centrist | 23,301 | 5.10% | 94.90% | 
| Third Way | Liberal | 11,850 | 0.00% | 100.00% | 
| Congressional Research Service | Centrist | 10,422 | 0.00% | 100.00% | 
| Institute for Policy Studies | Liberal | 5,850 | 0.00% | 100.00% | 
| National Bureau of Economic Research | Centrist | 1,750 | 0.00% | 100.00% |