A new report released today by the Center for Responsive Politics shows that national political spending by non-profit groups—mostly 501c4s, which do not have to disclose their donors—grew from about $5 million in 2006 to more than $300 million last year.

"These organizations can receive unlimited corporate, individual, or union contributions that they do not have to make public, and though their political activity is supposed to be limited, the IRS -- which has jurisdiction over these groups -- by and large has done little to enforce those limits. The report is searchable by donor and recipient," says the report from the Washington D.C.-based watchdog group.

The report is searchable by recipient and donor.