Wealthy Republicans dominate the list, led by casino magnate Sheldon Adelson and his wife, Miriam, who have contributed $25 million. Most of the Adelsons' money went to a super PAC aiding former House speaker Newt Gingrich's campaign for the GOP presidential nomination.
About $12 million of the megadonations came from labor unions, gearing up to support Obama and congressional Democrats this fall.
FreedomWorks For America, took in more than $300,000 from its non-profit arm this year, including in-kind donations of staffers' time, records show. "Everything we are doing is within the law," the super PAC's national political director Russ Walker said.
The group, which sent 60 volunteers to Saturday's Utah convention to help defeat Hatch, plans to remain active in the state and support his rival Dan Liljenquist in the June GOP primary, Walker said.
Freedom Path, meanwhile, has produced TV ads and mailers praising Hatch and fellow Utah GOP Sen. Mike Lee in recent months. But it is a non-profit that does not have to say who funds its activity. Attempts to reach its organizers Sunday were unsuccessful.
New reports filed with the FEC late last week also show that some contributions reported by super PACs can be hard to trace.
Restore Our Future, a pro-Romney super PAC, plans to correct its filings after providing an incorrect address for SeaSpray Partners, a limited liability corporation that gave $400,000 last month, spokeswoman Brittany Gross said. The original Palm Beach, Fla., address listed on the group's disclosure forms linked the contribution to a firm that has been inactive in the state since March 2011, according to Florida corporate records available on the Internet.
Gross attributed the mistake to a clerical error and declined to provide more information about that donor or others who have given to the super PAC. It received nearly $1.7 million from three limited liability corporations last month, about 19% of its March total.
Campaign-finance watchdogs argue that limited liability corporations, which are not required to publicly reveal their owners in some states, offer a way to obscure political givers' identities.
"There are well-known, legitimate LLCs," said Bill Allison of the non-partisan Sunlight Foundation, which tracks campaign money. "But we know very little about many of them. They are almost black holes in terms of campaign-finance disclosure."
More than 8 percent of Restore Our Future's contributions since Jan. 1, 2011 came from privately held limited liability corporations — the highest share of those donations flowing to the five super PACs that have raised the most money in the 2012 election.
About 2 percent of contributions to Priorities USA Action, a super PAC supporting Obama came from limited liability corporations during the same period, a USA TODAY tally shows.