A nonpartisan campaign watchdog group filed a complaint Monday against a pro-Doug Jones super PAC for failing to disclose its donors before the special election.

The Campaign Legal Center is accusing the Highway 31 super PAC of engaging in a "secrecy scheme to spend $4.2 million in the race" to aid Jones, a spokesman for the center told AL.com.

Highway 31's sole report to the Federal Election Commission before the election said it spent $1.15 million but raised no money. The group, headquarted in Birmingham, claimed its vendors lent them the money on credit.

Two weeks after the Dec. 12 special election between Jones and Republican Roy Moore, Senate Majority PAC -- a Democratic super PAC based in D.C. -- told the Associated Press that it was Highway 31's main backer.

The Campaign Legal Center's complaint to the FEC accuses Highway 31 of not diclosing that fact and misrepresenting its donors.

"Democrats talk the talk about supporting transparency in political money, but then national Democratic groups push aggressive new legal theories to undermine the transparency laws that are on the books," said Brendan Fischer, director, federal and FEC reform at the center. "Despite laws requiring that super PACs disclose their donors, Alabama voters went to the polls on election day without knowing who was backing Highway 31. This secrecy scheme cooked up by Highway 31 and its backers threatens to create a new disclosure loophole that will be exploited by billionaires and operatives supporting both parties, unless the FEC does its job and enforces our disclosure laws."