No one knows who funded the biggest spender in Pennsylvania's Senate Republican primary race this year, according to the Center for Responsive Politics, thanks to a chain of Super PACs and tax-exempt groups.

The Super PAC called Freedom Fund for America's Future spent a little over $175,000 in ads attacking Republican Senate candidate Tom Smith. That money accounted for more than half of the money spent during the Pennsylvania Senate Republican primary campaign.

At least $165,000 of the money came from another Super PAC, Fight for the Dream, and Fight for the Dream received all of its money from the tax-exempt organization Restore the Dream.

Super PACs are required to report their donors to the Federal Elections Commission, but Restore the Dream is registered as a 501(c)(4) nonprofit organization and is not required to disclose its sources of funding, effectively putting an end to the money trail.

"This was set up within federal election laws," Anthony Ferate, the legal counsel for Fight for the Dream, told the Center for Responsive Politics. "I would disagree that there's anything to question about transfers between super PACS. In fact, the Democrats are coordinating between their super PACs."

Democrats have challenged the tax-exempt status of some prominent Republican-leaning 501(c)(4) nonprofit organizations, including 60 Plus Association, Crossroads GPS, and Americans for Prosperity. The three groups have made millions of dollars worth of independent expenditures, which critics said have all be partisan, but have not registered as political action committees.

The Democrats argue that by registering with the IRS instead of the Federal Elections Commission, the three pro-Republican groups are “using secret money to subvert the democratic process.” The IRS requires that 501(c)(4) nonprofit organizations “operate primarily to further the common good.” The organizations are prohibited from running ads in support of or opposition to candidates for public office.