Democratic Rep. Chris Van Hollen, in his efforts to expose private donors in politics, has failed to report his own “secret money” in the form of tens of thousands of dollars’ worth of free legal assistance, according to a complaint to be filed with the Federal Elections Commission.

The complaint relates to the Maryland congressman’s 2011 federal court case arguing an FEC regulation is muddling and “eviscerating” a provision in the Bipartisan Campaign Reform Act.

The provision requires corporations, labor unions and others to disclose essentially everybody who gives donations that are used for campaign ads or so-call “electioneering communications.”

The case eventually reached the U.S. Court of Appels, which in January upheld the regulation.

Democracy 21, the liberal-leaning group that provided some of the pro bono legal work, said such services are “explicitly permitted” in the House Ethics Manual, according to The Daily Caller.

And Van Hollen filed the complaint through his congressional office.

However, the complainant, the Cause of Action Institute, points out that the congressman clearly states in his 2011 case filing that he is a candidate seeking re-election -- which would require him to report the value of the legal services on FEC disclosure statements.

“Van Hollen is a … candidate for re-election for Congress, voter, recipient of campaign contributions, fundraiser,” according to the congressman’s filing. “Van Hollen likely will be subjected to attack ads or other ‘electioneering communications’ financed by anonymous donors.”

Van Hollen has also made campaign finance reform a major part of his election platform, including this year’s Senate run.

“Chris has worked tirelessly to lift the toxic cloud of secret special interest spending that has flooded our election process and weakens our democracy,” according to his campaign website.

Cause of Action’s draft complaint states that Van Hollen has violated the Federal Election Campaign Act of 1971 by making campaign finance part of his “campaign rhetoric,” arguing he apparently “doesn’t live by the same rules and standards he would impose on others.”

A group spokesman said Wednesday the complaint has yet to be filed and suggest it likely won’t be in the near future.

The National Republican Congressional Committee tried to make a similar case in 2014 against Van Hollen but never filed a complaint.

The Center for Public Integrity’s David Leventhal, who reported on the case, said Wednesday the NRCC declined to explain its decision. But he suggested that such decisions typically involve weighing the likelihood of winning against time and money spent.

He also pointed out that the FEC ruling board is made up of three Democratic and three Republican appointees, who deliberate privately and frequently make split decisions, often many months later.

The complaint -- which legal observers describe as “dark money fighting dark money” -- also pits one of the wealthiest conservative donors against one the biggest liberal donors.

The non-partisan Cause of Action does not disclose its funding. But it purportedly has received millions through the non-profit group Donors Trust, which is connected to the billionaire Koch brothers.

The millions they have given in political donations largely go to conservative think tanks, Tea Party-affiliated groups and Republican candidates.

"This Koch Brothers-funded group is trying to silence Chris Van Hollen because he is leading fight against their secret money machine,” campaign spokeswoman Bridgett Frey told FoxNews.com Wednesday in a statement.

Democracy 21 is funded in part by Open Society Foundation, founded by George Soros, the billionaire investor who has given roughly $8 million so far this elections cycle to so-called “outside spending” groups that support liberal causes.

Van Hollen also received free legal work in his roughly 5-year-long case from the prestigious Washington, D.C. law firm Wilmer Hale.