WASHINGTON (AP) – Two watchdog groups have filed a complaint with the Justice Department about a nonprofit group funded by secret donors that has so far spent $8 million in advertisements promoting Republican presidential candidate Marco Rubio.

Democracy 21 and the Campaign Legal Center allege that the Conservative Solutions Project was set up primarily for Rubio’s benefit.

That would be a potential violation of tax law. Nonprofits face restrictions on political activity in exchange for being able to keep their donors’ identities from public view.

Two weeks ago, another watchdog group complained to the Internal Revenue Service that the Conservative Solutions Project was violating tax law.

Neither the IRS nor the Justice Department is confirming whether the matter is under investigation.

A Conservative Solutions Project spokesman says the nonprofit is doing nothing wrong.