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The Alabama Senate primary race between incumbent Republican Senator Richard Shelby and challenger Jonathan McConnell has seen an unexpected surge in outside spending-an influx that spotlights the growing role of undisclosed "dark" money in 2016, even in super PACs that supposedly must report their donors.

Citizens for a Sound Government, a political nonprofit that doesn't disclose its donors, funneled $400,000 in January to a super PAC that's supporting Shelby's re-election bid, according to the Montgomery Advertiser.

Citizen Super PAC, as the group is called, is run by a group of former Senate Republican political operatives and has so far spent more than $300,000 on ads supporting Shelby. The group has also shelled out $50,000 on ads opposing Shelby's toughest GOP primary contender, McConnell, a conservative Birmingham businessman who is running as a political outsider.

Ironically, the super PAC brands itself as a vehicle to amplify the voices of average citizens.

"Transparency is one of our founding principles. It's why we created the first platform that gives the American people the power to sponsor political speech directly," one of the super PAC's cofounders told the Montgomery Advertiser. But the Advertiser reported that the "PAC had received no online pledges to help finance the Shelby ads."

But none of the $400,000 that came to the PAC Citizens for a Sound Government can be traced, and a big chunk of the super PAC's receipts-$100,000-comes from a corporate PAC dubbed New Generation, which has direct ties to coal behemoth Murray Energy Corporation-hardly a mouthpiece for average citizens.

For its part, Citizens for a Sound Government has spent $413,000 on its own ads commending Shelby for opposing President Barack Obama's executive action to require criminal background checks of gun buyers, according to the Advertiser. The group's website says that its goal is "to promote policies that create a strong environment for job creation, enhance personal freedom, and generate fiscal responsibility."

It's not the only group backing Shelby with undisclosed money that has ties to prominent Republican operatives. As the Montgomery Advertiser reported earlier this week, One Nation, a group with ties to Karl Rove, spent $140,000 on radio ads supporting Shelby's opposition to Obamacare and to admitting Syrian refugees into the U.S.

Shelby is facing his first serious primary challenge since he took office in the 1980s, and the prospect of an insurgent outsider ousting a prominent Republican senator has attracted unprecedented levels of outside support for his re-election-so far nearly $900,000, according to the Center for Responsive Politics.

Dark-money spending has already reached record levels in the 2016 cycle, and experts expect that more is on the way. What's noteworthy about Citizens for a Sound Government's early investment in the Alabama Senate race is that it illustrates how super PACs, which theoretically are subject to full disclosure, can be used to obscure political funding sources.