WASHINGTON (AP) — Voters are beginning to learn about Republican presidential hopeful Marco Rubio.

What they're not learning, however, is who is paying to promote his candidacy.

The Florida senator is benefiting in unprecedented ways from a nonprofit group funded by anonymous donors. While other presidential candidates also have ties to secret-money groups, the Rubio arrangement is the boldest.

Every pro-Rubio television commercial so far in the early primary states of Iowa, New Hampshire and South Carolina has been paid for not by his campaign or even by a super PAC that identifies its donors, but instead by a nonprofit called Conservative Solutions Project. It's also sending Rubio-boosting mail to voters in those same states.

Rubio is legally prohibited from directing the group's spending, and he has said he has nothing to do with it. But there's little doubt that Conservative Solutions Project is picking up the tab for critical expenses that the campaign itself might struggle to afford.

Although Rubio is rising in national polls, his fundraising has so far been dwarfed that of by several rivals. For one, Jeb Bush and his super PAC had amassed $114 million — more than quadruple what Rubio and his super PAC collected — by the end of June.

Ahead of what is expected to be a disappointing fundraising report next week, Rubio's aides have stressed that their thriftiness gives them a competitive advantage over campaigns with more money.

Left unsaid is that a secret-money group is giving him at least an $8 million assist, according to advertising tracker Kantar Media's CMAG.

The candidate has presented himself as being opposed to such unaccountable money.

"I have always supported disclosure," Rubio said at a New Hampshire campaign stop last month, in response to a question about money in politics. "And I think that as long as people know who is giving you money, and why it is, people can make judgments on why you are doing what you are doing."

Conservative Solutions Project does not disclose its donors.

The group is spending more than $3 million on a commercial that shows Rubio, 44, speaking at the Iowa State Fair, according to CMAG information about advertising placements on broadcast, cable and satellite television.

"New ideas for a new age," a narrator says before ticking through a list of Rubio priorities: "Throw out the tax code, overhaul higher education, repeal and replace Obamacare."

That follows a $3 million summertime ad campaign by the same group that promoted Rubio's strong opposition to the Iran nuclear deal. Conservative Solutions Project also has reserved nearly $2 million in additional satellite TV time through Feb. 16, according to the advertising tracker.

Although numerous candidates may ultimately benefit from allied nonprofits, so far it appears that only the entities helping Rubio and Louisiana Gov. Bobby Jindal are advertising in the presidential race. America Next has spent about $380,000 boosting Jindal on TV, CMAG said. Bush also has a secret-money group in his corner, but it hasn't yet directly communicated with voters.

Nonprofits are the edgier cousins of super PACs. Both can accept unlimited amounts of money from wealthy donors, corporations and unions, but only nonprofits can keep those names a secret.

In exchange for that privilege, nonprofits are barred from making political activity their primary purpose. But gray area abounds. The two regulating agencies, the Federal Election Commission and the Internal Revenue Service, have been less than aggressive in pursuing potential violators.

"Congress, the Supreme Court and the public have all recognized that voters have a right to know who is spending money to try to influence them on Election Day," said Paul S. Ryan, senior counsel at the Campaign Legal Center in Washington. "Transparency is how we hold politicians accountable and make sure they're not in the pocket of their benefactors."

That's in line with public opinion: Seventy-five percent of voters, an equal share of Democrats and Republicans, said contributors to unaffiliated groups should be disclosed, according to a CBS News/New York Times poll in June.

The Conservative Solutions Project declined to say who gave it the $16 million it claims to have. Its donors will never be named in the IRS paperwork it is required to submit. And because of the filing schedule the group set for itself, the public will have to wait until mid-2017, well after the general election, to learn even basic information about its finances during the primary nomination fight.

Although it shares a name and key personnel with the Rubio-focused super PAC, Conservative Solutions PAC, its mutual spokesman, Jeff Sadosky, said the two are "very separate and distinct groups."

He said the nonprofit's work goes well beyond Rubio's presidential ambitions, pointing to a detailed study it did last year of voter behavior, available to anyone online. Additionally, Sadosky said, Conservative Solutions Project highlights on its website the work of other conservative leaders.

But its bent toward Rubio is apparent even there: Visitors to the site are immediately routed to a video of the Florida senator speaking, the same footage on television in early primary states.

___

Follow Julie Bykowicz on Twitter at: http://twitter.com/bykowicz