The watchdog groups that produced the study — Democracy 21, the Campaign Legal Center and the Center for Responsive Politics — said the data shows super-PACs are being used to undermine campaign finance law. 

Super-PACS are “vehicles for circumventing the candidate contribution limits which were enacted to prevent the corruption of federal officeholders and government decisions,” said Democracy 21 President Fred Wertheimer.

“Recent announcements by the Obama and Romney campaigns that high-level campaign staff will be appearing at super PAC fundraising events make clear that these super PACs are simply shadow candidate committees set up to evade the $2,500 candidate contribution limit,” said Paul RyanPaul Davis RyanDems: Ryan ‘sole impediment’ to DACA deal The Hill's 12:30 Report The Hill's Morning Report: Haley clashes with White House MORE, the Campaign Legal Center’s Federal Election Commission program director. 

“Million-dollar contributions to super PACs pose just as big a threat of corruption as would million-dollar contributions directly to candidates,” Ryan said.

The Red, White and Blue super-PAC that supports Rick Santorum saw nine of its 14 donors also contribute $2,500 to Santorum’s campaign. Similarly, five of the 18 donors to the pro-Newt Gingrich super-PAC Winning Our Future contributed the maximum to Gingrich’s campaign.

Ron Paul only received one $2,500 donation from four Endorse Liberty super-PAC donors and four maximum donations to his campaign out of seven donors to the Santa Rita super-PAC. 

"For most of these donors, $2,500 seems to be an almost insignificant amount compared to what they're contributing to the super PACs associated with the candidates," said Sheila Krumholz, the executive director of the Center for Responsive Politics, in the statement.

“Giving to the candidate is almost a perfunctory act on the way to giving a really substantial amount to a super PAC,” Krumholz said.

Donors are allowed to contribute up to $2,500 to an individual candidate’s campaign during the primaries and an additional $2,500 during the general election.