Conservative Solutions Project, a nonprofit group funded by secret donors that has so far spent $8 million in ads promoting Republican presidential candidate Marco Rubio, has drawn another complaint from Washington watchdog groups.

Democracy 21 and the Campaign Legal Center on Thursday asked the Justice Department to investigate the group, alleging that it was set up primarily for the benefit of the Florida senator.

That would be a potential violation of tax law, because nonprofits face restrictions on political activity in exchange for the benefit of being able to shield their donors’ identities from the public.

“The Conservative Solutions Project is flaunting the tax laws and flaunting the interests of the American people by using its improper claim of nonprofit tax status to keep secret the donors financing its campaign activities,” Democracy 21 president Fred Wertheimer said in a statement.

Two weeks ago, Citizens for Responsibility and Ethics in Washington complained to the Internal Revenue Service that the Rubio-boosting activity by Conservative Solutions Project violated tax law.

Neither the IRS nor the Justice Department would confirm whether investigations were underway.

Jeff Sadosky, a spokesman for Conservative Solutions Project, has said the nonprofit is doing nothing wrong and that Rubio was among the candidates the group has highlighted.

Yet the group’s emphasis on Rubio, one of more than a dozen major Republicans competing for the presidential nomination, is evident.

While the group mentions several conservative politicians on its website, only Rubio has appeared in its TV and radio ads — and there have been more than 3,000 on air so far.

In fact, Conservative Solutions Project is the second-largest advertiser in the 2016 race, according to Kantar Media’s CMAG advertising tracker.

In addition to the $8 million it had spent on Rubio-centric commercials through this week, Conservative Solutions Project has reserved nearly $2 million worth of advertising time on satellite television to begin airing next month and continue through the first three primary elections next year, information provided by CMAG shows.

The ad aired most frequently has the look and feel of a campaign message. It’s a 30-second clip of a speech Rubio gave to a conservative group, and is set to a soundtrack of soaring music and interspersed with patriotic images such as flags fluttering and a rocket launching.

“If ever there has been an era in human history tailor-made for us as a people, it is the 21st century,” Rubio says in the ad. “What is standing in the way are outdated leaders that refuse to let go of the past.”

This week, Citizens for Responsibility and Ethics in Washington filed a supplement to its IRS complaint noting that Conservative Solutions Project characterized all of its ads as “supporting Marco Rubio” when it signed contracts for the television time.

The nonprofit shares key leaders with a super political action committee specially designed to benefit only Rubio. Unlike the nonprofit, that similarly named PAC, Conservative Solutions PAC, must regularly disclose its donors to the public through the Federal Election Commission.

Sadosky is spokesman for both groups. Warren Tompkins, a South Carolina operative who is business partners with Rubio’s campaign manager, formed the nonprofit in 2014. He now leads the super PAC.

(AP)