Mr. Romney, who assailed Newt Gingrich in Florida last week for “working as a lobbyist and selling influence around Washington,” also got a major boost from some of the Republican Party’s top corporate lobbyists, who raised more than a million dollars in checks for Mr. Romney’s campaign during the last three months of 2011.

Patrick Durkin, a lobbyist for Barclay’s, raised over $400,000 in contributions. Bruce A. Gates, a lobbyist for Altria, the parent company of Philip Morris USA, raised $275,000. And Austin Barbour, a Mississippi lobbyist and nephew of the state’s former governor, Haley Barbour, raised $210,700 in contributions.

Restore Our Future raised at least $5.8 million from corporations during the last six months of last year, along with $12.2 million from individuals. American Crossroads raised $4.6 million from corporations and $7 million from individuals. Priorities USA and two other Democratic-leaning super PACs raised about $1,835,000 from individuals, $1.3 million from political action committees affiliated with labor unions and other groups, and about $415,700 from other organizations.

Image Harold Simmons donated to “super PACs” supporting Republican presidential candidates. Credit... Flor Cordero/Reuters

Groups supportive of each party employed a technique that allows them to cloak the identities of many of their donors. Those groups, including Crossroads and Priorities USA, have affiliates that are organized as nonprofit organizations known as 501(c)(4) groups, which can raise unlimited money but do not have to reveal their donors. Donors wishing to remain anonymous have the option of making their contributions to those nonprofit groups, which raised tens of millions of dollars in 2011, according to officials at the groups.

“While we now know some names of some people giving megabucks, we know nothing about the funders of the nonprofits,” said Ellen S. Miller, the executive director of the Sunlight Foundation, which advocates for greater transparency in political giving. “We don’t know what we don’t know.”

Even donors who chose not to give via nonprofit affiliates can find ways to guard their identities by giving through a limited liability corporation or other entity that is hard to trace.