Adopting generic names like Restore Our Future and Red, White and Blue, they can accept unlimited donations from corporations, unions or individuals.
They have emerged since a 2010 Supreme Court ruling struck down limits on how much unions and corporations can spend on elections, fully opening the floodgates to outside spending.
Senator John McCain, who has long advocated for limits on campaign donations, said the court's ruling was "one of the worst decisions I have ever seen".
He added: "I predict that there will be huge scandals associated with this huge flood of money."
Michael Malbin, executive director of the Campaign Finance Institute, which advocates for funding by small donors and tax revenue, said the groups were "all but arms of the candidates".
"It would be a lot healthier for democracy if there were alternative sources of funding rather than going to a few rich people to whom candidates are beholden," he said.
President Barack Obama will also benefit from Super-PACs when his campaign against whoever is the Republican nominee begins in earnest.
But he also will be well financed by conventional means. His campaign announced yesterday that it had raised more than £44 million in the final quarter of 2011, about £28 million more than Mr Romney.
It has been forecast that he could in 2012 become the first $1 billion candidate.