A super PAC that spent millions of dollars to support Democrat Doug Jones in Alabama’s Senate race was accused Monday of breaking campaign finance law by engaging in a “secrecy scheme,” failing to disclose donors until after the election last December.

A complaint against the Highway 31 political action committee was filed with the Federal Election Commission by the Washington D.C.-based nonpartisan watchdog group the Campaign Legal Center. The complaint alleged that the political action committee used lines of credit from vendors to avoid disclosing that it was backed by well-funded national Democratic groups.

“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,” Brendan Fischer, a spokesman for the Campaign Legal Center, told Al.com. “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.”

Highway 31 spent $4 million in the 2017 special election where Jones defeated Republican Roy Moore, whose candidacy was bogged down by allegations of sexual misconduct. It was disclosed after the election that the PAC received more than $3 million from the Senate Majority PAC, which works to elect Democrats to the U.S. Senate and $910,000 from Priorities USA.

In a statement to Fox News, the Campaign Legal Center called the disclosure “interesting, given Doug Jones’s repeated attempts to distance himself from major national groups outside Alabama — and the Democratic platform of supporting the transparency of political money.”

Highway 31 has maintained it fulfilled all reporting requirements. Adam Muhlendorf, a spokesman for Highway 31, on Monday dismissed the allegations.

“This is yet another frivolous complaint. Highway 31 has closed its books,” Muhlendorf said.

Highway 31, named after a roadway crossing much of the state, emerged as a major player in the Alabama election. Many of its advertisements and mailers focused on accusations against Moore of sexual misconduct involving teenage girls decades ago. Moore denied the misconduct accusations.

The complaint said that soon after forming, Highway 31 got a line of credit from out-of-state vendors to avoid having to report donors, and speculated that Senate Majority PAC, or others, guaranteed that the vendors would get paid.

“It is hard to believe that the vendors here would, in the ordinary course of business, extend hundreds of thousands of dollars in credit to an entity that was formed just weeks earlier and had a total of $0 in its bank account,” the complaint read.

Fischer said he was concerned that, if not stopped, it would create a new “loophole” that political groups on both sides of the aisle could repeat.

The complaint also said the PAC reported a contribution from Priorities USA Action that the group did not report making.

Muhlendorf said that was a “minor inaccuracy that will be corrected.”

Fox News' Judson Berger and The Associated Press contributed to this report.