Several attend the exclusive, secretive gatherings of wealthy conservative donors hosted twice a year by the billionaire Koch brothers. Many move in the same social or political circles: Sheldon Adelson, a billionaire casino executive who is close to Mr. Gingrich, serves on the board of the Republican Jewish Coalition with Paul Singer, a hedge fund executive and a top contributor to Restore Our Future.

Some of the million-dollar-plus donors, however, are relatively new to the world of big-league political giving and appear to be motivated by personal connections to particular candidates. Paul B. Edgerley and his wife, Sandra, for example, together gave $1 million to the pro-Romney super PAC. Mr. Edgerley is an executive at Bain Capital, Mr. Romney’s former firm.

A few of the megadonors gave through limited liability companies, shielding their identity. One $1 million donation to Restore Our Future came from F8 LLC, a company whose listed address in Utah leads to an accounting firm. A charitable foundation linked to Sandra N. Tillotson, co-founder of the skin care company Nu Skin, uses the same address. Ms. Tillotson was reimbursed by Restore Our Future in July for what appeared to be costs associated with a fund-raiser at her New York apartment. But Ms. Tillotson said in an e-mail Wednesday that she did not know who the owner of F8 LLC was and had not made a donation backing Mr. Romney's campaign.

But the superdonors all have one thing in common: they are by definition deep-pocketed, willing and ready to give far more than the $2,500 checks that donors to candidates are limited to writing. Some of them have almost singlehandedly financed super PACs that support favored candidates.

Peter Thiel, the co-founder of PayPal, has given Endorse Liberty, a super PAC supporting Representative Ron Paul of Texas, at least $2.6 million, the bulk of the group’s donations so far. Mr. Adelson and his family have contributed over $10 million to Winning Our Future, the group supporting Mr. Gingrich.

“I might give $10 million or $100 million to Gingrich,” Mr. Adelson said in a coming profile in Forbes magazine.

The motivation and logistics behind these large contributions remain something of a mystery. Most megadonors and the campaigns they support are reluctant to talk about the hidden scaffolding of high-level political giving. But glimpses of it emerge in public records.