Conservative Solutions Project, an outside group promoting Republican Marco Rubio's presidential campaign, has spent nearly $8.5 million in TV ads -- making it the second-biggest advertiser in the 2016 Republican race so far. But the group’s apparent support for a single presidential candidate has raised questions about the advertisements’ legality.

The ads, which have aired in the early nominating states of Iowa and New Hampshire, have featured Rubio denouncing the Iran deal and delivering one of his early political speeches in 2015 -- months before Rubio’s own campaign started running its first TV ad this week.

And unlike a Super PAC, Conservative Solutions Project doesn't have to disclose its donors because it exists as a tax-exempt social welfare group under section 501(c)(4) of the tax code.

But it's precisely that tax-code designation that has campaign-finance watchdogs alleging the Conservative Solution Project ads are illegal -- because they are benefitting an individual presidential candidate rather than advancing the social welfare.

“I think they’re breaking the law,” Paul S. Ryan of the Campaign Legal Center tells NBC News.

Earlier this month, the Campaign Legal Center and Democracy 21, another campaign-finance watchdog, asked the Justice Department to launch an investigation into Conservative Solutions Project.

“The publicly available facts indicate that Conservative Solutions Project is little more than a single-candidate 501(c)(4), with no other mission than to advance the presidential aspirations of Marco Rubio,” the Campaign Legal Center said in its statement calling for an investigation.

The statement added, “501(c)(4) ‘social welfare’ groups by statute must [promote] the common good and general welfare of the people of the community as a whole’ rather than an individual candidate for political office.”

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Officials at Conservative Solutions Project deny that the organization is supporting Rubio’s presidential candidacy.

“Conservative Solutions Project, as a 501(c)(4), is not about any one specific elected official or candidate,” spokesman Jeff Sadosky told NBC News. “ It’s focused on issue education and helping the conservative movement most effectively communicate with American families so that we win the battle of ideas and are able to enact conservative solutions to the problems they face.”

A spokesperson for Rubio’s presidential campaign declined to comment. (Campaigns are barred from strategizing with 501c4s and super PACs.)

Yet there are some clear ties between Conservative Solutions and efforts to benefit Rubio’s presidential candidacy.

For starters, Rubio is the only 2016 presidential candidate featured in any of the organization’s TV ads that have aired in the early nominating states of Iowa, New Hampshire and South Carolina, as well as on national cable.

One Conservative Solutions Project ad features Rubio -- and Rubio alone -- giving a 2012 speech. “In this 21st Century, who will win? Whose idea will continue to spread across the globe?” Rubio asks in the ad. “Will it be our ideas of freedom, democracy and free enterprise? Or will it be their ideas of state-owned enterprises and totalitarianism?”

An earlier ad from the summer criticized the nuclear deal with Iran. “Congress can stop it,” the ad’s narrator says. “Marco Rubio is leading the fight.”

The sole Conservative Solutions Project TV ad to mention any Republican other than Rubio is one that also names Sen. Mike Lee, R-Utah, who has co-authored a tax proposal with Rubio. “Sens. Marco Rubio and Mike Lee have a bold tax-reform plan,” that ad goes.

Lee, however, isn’t running for president.

But Sadosky, the Conservative Solutions Project’s spokesman, responds that the organization’s website has a focus beyond just Rubio. “As you can see on our website, to which our TV and online advertising are driving attention, that’s not about any one specific elected official or candidate, it’s about how best to address the problems facing our nation,” he says.

In another indication of the group’s support of Rubio, Conservative Solutions Project is directly related to a pro-Rubio Super PAC with a similar name -- Conservative Solutions PAC -- and it shares the same leadership (South Carolina Republican operative Warren Tompkins) and same spokesman (Jeff Sadosky). And the treasurer for Conservative Solutions is Robert Watkins, while the treasurer for Conservative Solutions PAC is his wife Nancy Watkins.

The website for Conservative Solutions PAC is all about Rubio’s presidential campaign.

“Absolutely, the two groups are related,” Sadosky told National Journal back in April. Yet Sadosky added to the publication: “But they are separate and distinct entities. One is focused on supporting Marco Rubio’s potential presidential campaign, and one is focused on issue education.”

And maybe most striking of all, NBC News has obtained at least two advertising filings with the Federal Communications Commission in which Conservative Solutions Project appears to describe its advertising as being on Marco Rubio’s behalf.

Dann, Caroline (206104031)

Dann, Caroline (206104031)

Sadosky of Conservative Solutions says it was the TV station -- and not his organization -- that incorrectly filled out the form. “Your form is incorrect and was prepared and published without input from or any knowledge by CSP,” Conservative Solutions Project’s lawyer said in a letter to the station, New Hampshire’s WMUR, earlier this month.

But media-buying sources tell NBC News that these forms are the advertiser’s responsibility. And if there was a mistake on the forms, both sides signed off on the contract.

A more recent Conservative Solutions Project contract with WMUR no longer mentions Rubio’s name.

What is a 501(c)(4)?

Under tax law, a 501(c)(4) must operate “exclusively to promote social welfare,” and that “an organization must operate primarily to further the common good and general welfare of the people of the community.”

In addition, an organization that “is primarily benefiting a private group rather than the community … does not qualify as a section 501(c)(4) organization,” according to the IRS website.

501(c)(4) groups don’t have to disclose their donors, but the tradeoff is that they must adhere to restrictions in their political activity. Critics charge that Conservative Solutions is reaping both benefits -- by ignoring the political restrictions while operating with secret money.

Indeed, campaign-finance watchdogs charge that by appearing to back Rubio, Conservative Solutions Project is violating Internal Revenue Service regulations, not campaign-finance law.

The most blatant offender?

Since the 2010 Citizens United decision by the U.S. Supreme Court, these watchdog groups have filed complaints against politically oriented 501(c)(4)s in the past, including Republican political strategist Karl Rove’s Crossroads GPS and Priorities USA, which backed President Barack Obama’s re-election campaign.

But Democracy 21’s Fred Wertheimer said Conservative Solutions Project is the most blatant offender, because it is spending millions on political advertisements in promoting Rubio.

“We filed a complaint against [Conservative Solutions Project] because it was the most blatant example … and are just kind of blatantly ignoring the laws,” Wertheimer said.

The challenge, however: Enforcement of the laws is mostly nonexistent.

“The huge problem is we don’t have any enforcement of the laws and all of the political campaigns know this,” Wertheimer added.

The Internal Revenue Service has shown little resolve to adjudicate such challenges. And that’s why the National Legal Center and Democracy 21 hope that the Justice Department will investigate.

“If these apparent violations are left unchallenged, they will quickly be emulated by candidates for Congress and soon by candidates for state and local office as well,” the groups said in their November statement.