With no quick end to the nominating contest in sight and many major Republican donors sitting on the sidelines or putting their cash into the Republican National Committee instead of the party’s primary, super PACs are increasingly taking up the slack for the campaigns. And their ability to raise unlimited amounts of money makes them ever more critical to the candidates they support.

Restore Our Future, the super PAC supporting Mr. Romney, spent more than $12 million in February, most of it on advertisements attacking his rivals as he battled in seven primaries and caucuses that month, according to campaign filings released on Tuesday. That followed close to $14 million in spending on Mr. Romney’s behalf in January.

The group raised about $6.4 million — more than some of the candidates — from businesspeople and entrepreneurs who have long financed conservative causes, along with a small group of Wall Street executives who are also among top donors and fund-raisers for Mr. Romney’s campaign. Nearly half of that amount came from a single donor: Bob J. Perry, a Texas homebuilder who is among the leading Republican donors in the country and who has financed groups like Swift Boat Veterans for Truth, an independent group that broadcast advertisements against Senator John Kerry, the Democratic presidential candidate in 2004.

Two other donors gave half a million dollars each to Restore Our Future: A. Jerrold Perenchio, a former chairman and chief executive of Univision, and David Humphreys, a Missouri businessman. Eleven donors gave $100,000, among them Harold Simmons, the leading super PAC donor in the country; Ken Griffin, an investor with the Chicago-based hedge fund Citadel; Griffith Harsh IV, the husband of Meg Whitman, a former chief executive of eBay; and Henry Kravis, a leveraged buyout investor.

A super PAC helping Mr. Gingrich, Winning Our Future, reported $5.5 million in contributions from Sheldon Adelson, a casino billionaire, and his family — more than twice as much as Mr. Gingrich raised himself — bringing their total contributions to the group to close to $16 million. Mr. Simmons, who has given to multiple candidates and super PACs, also gave the group $100,000. The group raised a total of $5.7 million in February.