Marco Rubio's been called many things this primary season. A rising star. The GOP establishment's last, best hope. Sweaty.

Let's add one more to the list: He's the dark money candidate.

Rubio has benefited from anonymous, undisclosed cash to a degree that's unprecedented for a modern presidential primary contender. Indeed, the vast majority of ads aired to promote Rubio so far this year have been funded by a single group — one that won't reveal its funders.

And Rubio is unique in this. According to data from NBC News and SMG Delta, ads promoting every other candidate in both parties have been overwhelmingly funded either by the candidates themselves or by Super PACs (which disclose their donors).

At least $8.4 million has been spent on pro-Rubio dark money ads so far

This pro-Rubio group — the Conservative Solutions Project, a nonprofit — keeps its fundraising sources anonymous. It can raise, and has been raising, tons of money, and spending it on pro-Rubio ads, without telling the public where that money came from. And as of early November, it was the second-largest advertiser in the entire 2016 race, according to the Associated Press's Julie Bykowicz.

Campaign finance watchdogs are appalled. "Never before" have we seen an anonymously funded group like this "seemingly shoulder sole responsibility for a presidential candidate’s TV advertising in multiple early presidential primary states," says Paul S. Ryan of the Campaign Legal Center. "It's a huge new problem." (A Rubio spokesperson didn't respond to a request for comment on the topic.)

There have been various (excellent) one-off reports about all this over the past few months — first from National Journal, then from the New York Times, then from the Associated Press, then the Times again. Yet this highly unusual situation hasn't really become central to the media narrative of Rubio's candidacy.

It should. At least $8.4 million has been spent on these pro-Rubio ads so far. Who provided the money?

And what might these generous donors hope to get in return?

Why dark money groups have avoided presidential primaries — until now

"Dark money" groups — nonprofits that don't have to publicly disclose their funding sources — have become increasingly popular among wealthy donors over the past few years, on both the left and right.

But there's a catch. Most groups like these were created under Section 501(c)(4) of the tax code, and that means they have to be "operated exclusively to promote social welfare," according to the IRS. So they're not supposed to exist just to help candidates win elections — and they're especially not supposed to just help one candidate win one election. So while some dark money groups have seriously pushed the limits of this definition, they've generally been hesitant to get involved in presidential primaries, due to concern about legal exposure.

That's changing this year. Despite the potential legal risk, the allure of extra anonymous cash has proved too tempting for the many candidates' operations to turn down. By July, supporters of eight GOP candidates — including Rubio, Jeb Bush, Mike Huckabee, and Rick Santorum — had also set up dark money groups.

"These nonprofits are being created by presidential campaigns for one reason — and that is to allow secret money to be given to benefit them," says Fred Wertheimer of Democracy 21.

Yet the other candidates' nonprofits have, for the most part, not seemed particularly noteworthy so far. That’s because they’ve stayed away from one of the biggest and most visible expenses of traditional campaigns — TV advertising. (The Jeb Bush–affiliated dark money group, for example, has described itself as a policy development operation.)

Rubio's dark money group takes us the furthest into uncharted territory — by spending millions on TV ads

The Conservative Solutions Project describes itself as a nonprofit focused on "issue education" — not on the 2016 elections. But it’s been spending heavily on TV spots that look a whole lot like ordinary campaign ads for Marco Rubio.

Take the one below, which shows Rubio speaking about American greatness for nearly 30 seconds, over soaring music and patriotic imagery like billowing American flags — you'll notice that specific issues are absent from the spot:

The ad never advocates that the viewer vote for Marco Rubio, and indeed never even mentions that there's a campaign going on. But it's clearly trying to make viewers feel really, really good about this Rubio guy while he's running for president.

The other ads are similar. Some have a policy fig leaf — for instance, the group's first ads trashed Obama's nuclear deal with Iran and urged people to tell their senators to "join Marco Rubio" in opposition to the deal. But they were clearly aimed at promoting Marco Rubio's opposition to the deal as a special and noteworthy thing, even though every Republican senator and GOP presidential candidate also opposed it.

In an email, Jeff Sadosky, a spokesperson for the Conservative Solutions Project, said the purpose of the group's ad spending was "to drive opposition to the President's dangerous deal with Iran and to highlight effective methods of communicating with American families so that we win the battle of ideas and are able to enact conservative solutions to the problems they face." He added that the group also supports "an overhaul of our nation’s tax code" and "restoring our military and America’s standing in the world."

But much of the group’s spending has been targeted at the first primary and caucus states: Iowa, New Hampshire, and South Carolina. So, though the group can't say it, it’s apparent to basically everyone that the true purpose of these ads is to introduce Rubio, his oratory, and some of his policy views to voters at the start of primary season.

This group is really, really closely tied to Rubio's team

Though the Conservative Solutions Project calls itself an independent group, it’s effectively an extension of the operation trying to elect Rubio president. Consider:

It has the same root name and some of the same consultants as a pro-Rubio Super PAC called "Conservative Solutions PAC."

Its current president is Pat Shortridge, a top adviser to Rubio's 2010 Senate campaign. Its founder was J. Warren Tompkins, who is currently running that pro-Rubio Super PAC but still sits on the Conservative Solutions Project's board.

Though founded in January 2014, the group kept a very low profile until just before Rubio was to announce his presidential campaign, when National Journal's Scott Bland first reported its existence.

The group decided to voluntarily reveal how much money it had raised in July, right around the time campaigns and Super PACs were disclosing their fundraising — in an apparent attempt to make the overall haul of Rubio’s operation look more impressive.

Perhaps most significantly — in contrast to several other major candidates, Rubio’s own campaign hadn’t paid for any TV ads at all yet as of mid-November. (It just released its first ad on Monday.) This hesitance was likely because the campaign knew the Conservative Solutions Project had Rubio covered on the airwaves. "The candidate can stand back and not make a single TV ad buy, knowing that his ad campaign is being bankrolled by this dark money group," says Ryan of the Campaign Legal Center.

Even Jeb Bush's allies — surely no opponents of big money in politics — are scoffing at all this, albeit a tad opportunistically. "It’s cynical to run as the creature of new, fresh — while it’s all secret dark money," Bush Super PAC strategist Mike Murphy told Sasha Issenberg last month. That same week, Bush communications director Tim Miller tweeted about Rubio's "secret money TV ads." (Bush, of course, raised vast, record-breaking sums of money for his Super PAC this year — but, again, those donors had to be disclosed.)

Campaign finance reformers are even blunter. "This is the most dangerous form of political money that exists," says Fred Wertheimer of Democracy 21. "Unlimited secret contributions given to directly benefit an officeholder or candidate is the most dangerous and potentially corrupting money in American politics."

Accordingly, complaints against the nonprofit have been filed with the IRS (by Citizens for Responsibility and Ethics in Washington) and the Justice Department (by Democracy 21 and the Campaign Legal Center). "My view is that this group is likely violating federal campaign finance law — any group trying to influence the election is supposed to register as a PAC and disclose its donors," says Ryan.

In his email to me, Sadosky said that "DC’s left-wing elites are incredibly afraid" of the group’s "positive conservative message focused on solutions." Asked if the Conservative Solutions Project had any plans to disclose its donors, he only said that it would file its required forms with the IRS — forms that are not made public.

Rubio has a history of being very close to his top donors

Dark money has its defenders. Bradley Smith, a former Republican FEC commissioner who has authored the article "In Defense of Political Anonymity," told me that while he hadn't followed this particular case too closely, "I always wonder, what do people think they'll learn?"

Disclosure of particular donors' identities, in Smith's view, tells us little about "what the candidates actually think." He adds, "If we put a bunch of names out there that nobody’s ever heard of, it wouldn’t make much difference; I doubt that anybody would find much that’s interesting."

Furthermore, Smith adds, the non-disclosing money will, in the end, be a small part of the overall spending on the presidential election — a drop in the bucket. Indeed, Rubio has recently won some public endorsements from billionaires such as Paul Singer and Frank VanderSloot. If he wins over the GOP establishment, more money will pour in, making the Conservative Solution Project's spending look quaint.

Secret money in presidential primary campaigns "could become the new normal"

But politicians owe the most to people who've had their backs the earliest. And Rubio has shown a propensity to remember his financial backers in the past.

For instance, in 2008, the Miami Herald's Marc Caputo reported on how Rubio — then speaker of the Florida state House — "quietly slipped tough-to-spot language" into a bill to help "a friend and political money-man bid on a major fuel contract in a $265 million turnpike overhaul proposal." (The donor, Max Alvarez, had previously said Rubio was "like a son to him.")

And Rubio's closeness with donor and billionaire auto dealer Norman Braman is even more remarkable. As the New York Times's Michael Barbaro and Steve Eder chronicled, Rubio fought hard to win state funding for both a cancer center and a genomics center that were named after Braman. Afterward, Rubio went to work for Braman's company as a lawyer for a few months, Braman gave $100,000 to fund Rubio's teaching salary at Florida International University, and Rubio's wife, Jeanette, became a paid adviser to Braman's charitable foundation despite little experience in philanthropy. This year, Braman gave $5 million to Rubio's Super PAC.

Furthermore, there's the precedent that's being set — which stretches far beyond Rubio. "If the Conservative Solutions Project gets away with this, then it will become the new normal," says Ryan. "Candidates will offer a dark money option to any of their billionaires who don’t want to be publicly associated with the candidate for whatever reason." Billionaires could be able to secretly fund these operations with impunity.

Earlier this year, Rubio was asked at a New Hampshire town hall about all the outside money flowing into the election. "Full disclosure and sunlight into all these expenditures is critical to getting to the root of this problem," Rubio replied, according to the New York Times.

"As long as you know who’s behind the money and how much they’re giving and where they’re spending it," he continued, "I think that’s the sunlight that we need."