A Washington watchdog group says in a complaint filed with the IRS that a secret-donor nonprofit group backing Marco Rubio is violating tax law.

Conservative Solutions Project is spending millions of dollars on pro-Rubio television commercials and mailings to voters in early primary states as the Florida senator seeks the Republican presidential nomination.

The Rubio campaign itself is not yet airing TV ads.

The watchdog group Citizens for Responsibility and Ethics argues in a complaint filed Tuesday that Conservative Solutions is operated primarily to help elect Rubio and is therefore breaking the law. Unlike campaigns and super PACs, nonprofits do not have to disclose their donors.

But in exchange for that privilege, tax law requires that they stick primarily to nonpolitical activities.

(AP)