Following his back-to-back wins in Iowa and New Hampshire, it isn't a stretch to say that Mitt Romney is being widely hailed as the presumptive Republican nominee for the presidency.

While almost every GOP presidential candidate has experienced at least a temporary surge in the polls, Romney, who has been endorsed by high-profile members of the Republican establishment, has bounced back every time, putting down firestorms by Michele Bachmann, Rick Perry, Herman Cain and, in New Hampshire,  Rick Santorum.

As the primary race shifts to South Carolina, Romney continues to lead the pack. Although both Santorum and Ron Paul experienced significant surges following the Iowa caucuses, Romney effectively only has one real challenger left: Newt Gingrich.

Romney's super-PAC v. Gingrich's super-PAC

Gingrich may have fallen to the sidelines after disappointing finishes in Iowa and New Hampshire, but he is still the only candidate with enough financial support to compete on near Romney's scale -- and the Romney campaign knows it.

Restore Our Future, a pro-Romney political action committee, has spent more than $7.5 million in media buys, print advertisements and direct mail campaigns opposing a Gingrich presidency this election cycle, according to the Center for Responsive Politics. According to data available from the Web site, the Romney super-PAC has not spend any money targeting any of the other GOP candidates, instead focusing its resources on anti-Gingrich attack ads, aside from the $101,538 the group dedicated to pro-Romney media buys in the days following the Iowa caucuses.

Between Jan. 2 and Jan. 11, Restore Our Future spent more than $2 million on anti-Gingrich advertisements, a move that likely contributed to the former speaker's fourth-place finishes in both Iowa and New Hampshire.

Meanwhile, the similarly named pro-Gingrich super-PAC Winning Our Future has spent more than $2.6 million on anti-Romney television, Internet and radio advertising that also serve as Gingrich promotional spots, while donating about $1.2 million toward pro-Gingrich ads. The only other supportive independent expenditures for Gingrich this election cycle came from Strong America Now, a non-profit organization that describes itself as a bipartisan grassroots effort focused on eliminating the national debt and deficit using a proven waste-elimination process called Lean Six Sigma.

Founder Mike George reportedly formed the Strong America Now super PAC on Nov. 1, and it has since expended $84,719 on behalf of Gingrich.

In addition to backing from Restore Our Future, the Citizens for a Working America PAC has spent $475,000 in support of Romney this election cycle. Although the group initially backed Bachmann, Federal Election Commission data shows it made its almost half-million ad buy in support of Romney on Dec. 24, by which point the Bachmann campaign was already struggling to remain relevant.

While only the Romney PAC has devoted funding to anti-Gingrich media buys, Romney has been targeted by a variety of organizations. In addition to the Gingrich-supporting Winning Our Future, the liberal-leaning organizations Priorities USA Action and and DGA Action, a super PAC affiliated with the Democratic Governors Association, have put thousands of dollars toward anti-Rommey advertisements, indicating that the left perceives Romney as its primary threat this election year.

What about Santorum, Paul, Perry and Huntsman?

Aside from Romney and Gingrich, super PACs affiliated with the other Republican candidates have focused their resources on promoting their candidates, instead of disparaging their competition.

Available FEC data shows the pro-Paul Santa Rita Super PAC and Endorse Liberty have spent a total of $1,344,126 on Web, television and social media advertising this election cycle. While the Romney and Gingrich super PACs have not devoted resources to oppose Paul, DGA action has spent small sums on anti-Paul online advertisements on Facebook, Google and Twitter. The conservative National Organization for Marriage has spent more than $130,000 on automated phone calls and online advertisements attacking Paul's position on marriage.

The super PAC's Leaders for Families and Red, White and Blue have spent about $945,000 on pro-Santorum advertisements, although expenditures dried up on Jan. 7. Meanwhile, DGA Action -- a grassroots network -- has spent just more than $2,000 on online advertisements opposing Santorum.

While the FEC reports the pro-Perry group Make Us Great Again has devoted about $3.8 million toward pro-Perry advertisements this election cycle, it has not made any independent expenditure in the past two weeks. Our Destiny PAC, formed to support Jon Huntsman, has not made expenditures for pro-Huntsman media buys since Jan. 6.

Who's Money is it?

According to Sheila Krumholz, the executive director of the Center for Responsive Politics, the GOP primary race has been marked by an unusual lack of transparency. The U.S. Supreme Court's 2010 ruling in Citizens United v. Federal Election Commission triggered the creation of super PACs, which are allowed to collect unlimited sums of money from individuals, corporations, unions and trade groups and then use that funding toward expenditures calling for the support or defeat of a particular candidate.

The super PACs are not required to publicly disclose their donors via financial disclosure reports until Jan. 31 -- after the primary races in South Carolina and Florida. The groups have not disclosed the names of their donors since last July, meaning the public will not know who bankrolled the political advertisements until the crucial stage of the Republican primaries has passed.

Over the past few weeks, voters in early primary and caucus states have been deluged by political advertising, Krumholz wrote in an op-ed column for The New York Times. But even as they choose from among the Republican presidential candidates, voters haven't been able to find out who is really behind the spots.

What we do know is that Gingrich, despite his precipitous fall in Iowa, is likely to stick around for a while. The billionaire casino magnate Sheldon Adelson recently donated $5 million to the Gingrich super PAC Winning Our Future, reportedly the single largest donation that could directly aid a candidate in U.S. history. More mega-donors are expected to be revealed when the super PAC filings are released ahead of the FEC deadline at the end of the month.