A new study of donations during the 2012 election cycle has revealed scores of "double-duty donors" who are supporting their chosen candidates by giving millions of dollars to Super Pacs as well as the much smaller maximum amount allowed as a direct campaign donation.

The move is a legal way of dodging campaign finance laws which strictly limit the amount any one person can give to a candidate's campaign to $2,500 during the nomination race and another $2,500 for a general election. However, donations to Super Pacs can be unlimited, so donors can use them to continue giving huge amounts of cash in support of their favoured candidate.

Critics say the phenomenon of double-duty donors undermines the entire point of limiting direct campaign contributions, which is intended to remove the direct threat of corruption from American politics. However, now a generous benefactor can simply switch their donating to a Super Pac that supports their candidate once they have maxed out their individual contribution.

"This is a gaping loophole. You almost ask: why would anyone bother giving a direct campaign contribution any more?" said David Vance, a spokesman for the Campaign Legal Center which together with Democracy 21 and the Center for Responsive Politics compiled a list of double-duty donors from 2011.

That research showed that 172 people who donated money to the Mitt Romney-supporting Super Pac Restore Our Future had also maxed out their $2,500 individual donations. Five of them gave millions of dollars each to Restore Our Future, including hedge fund financiers Paul Singer, John Paulson and Julian Robertson. Double-duty donors made up some 84% of Restore our Future's donor base. But Romney's organisation is far from alone. Nor is it just a Republican issue.

Former Pennsylvania senator Rick Santorum, who has emerged as Romney's main competitor, also has a Super Pac that supports him called Red, White and Blue. Of that Super Pac's 14 individual funders, nine of them also gave the maximum $2,500 individual donation. On the Democratic side of the political divide some 15 out of 55 people who gave money to a Barack Obama-supporting Super Pac, called Priorities USA Action, were double-duty donors. They included Jeffrey Katzenberg, co-founder of DreamWorks Studios, who gave $2m dollars.

The phenomenon exposes one of the biggest open secrets of US politics: that Super Pacs are essentially shadow campaigns existing solely to dodge campaign finance laws. They came into existence after a controversial 2010 supreme court decision allowed them to get unlimited donations but legally barred them from officially co-ordinating with any specific campaign. However, many are candidate-specific in their support and often run by former aides or close associates of those candidates.

"Presidential candidate-specific Super Pacs are simply 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.

Huge amounts of money are now flowing into Super Pacs of all shapes and sizes. Not included in the study are the donations of casino magnate Sheldon Adelson and his wife, Miriam, who gave $10m to a Newt Gingrich-supporting Super Pac last month.

Critics say that the reason wealthy individuals give so much money to candidates is because they seek to influence politics and policy and are willing to use their fortunes to do so. "If you are a candidate they have given that much money to, it would be really difficult to not take their call. You can't tell me there is not corruption or at least the appearance of corruption when you can give a million dollars or more to a candidate-specific Super Pac," said Vance.

But hefty donations can also bring unwanted media attention down on a major donor to cause problems for a campaign. Santorum's biggest Super Pac funder, millionaire investor Foster Friess, who is also a double-duty donor, recently made extreme remarks about contraception during a TV interview.

The gaffe then caused a firestorm of controversy for the Santorum campaign.

Meanwhile, Adelson's massive support for Gingrich has turned a focused media spotlight on the usually media-shy businessman. This month he gave a rare free-ranging interview to Forbes magazine in which he explained his donations and said he might end up giving $100m to Gingrich's cause. "I'm against very wealthy people attempting to or influencing elections. But as long as it's doable I'm going to do it... I have my own philosophy and I'm not ashamed of it," Adelson told the magazine.