A group founded by Sen. Bernie Sanders (I., Vt.) was hit with a complaint alleging that it violated campaign finance laws by accepting contributions that exceed federal limits.

Our Revolution, a nonprofit established by Sanders, is at the center of a new complaint filed Wednesday to the Federal Election Commission by watchdog group Common Cause. An attorney for the watchdog said the pro-Sanders group violated the law by backing the Vermont senator's presidential candidacy without registering with the FEC.

"Because Sanders set up Our Revolution and they have raised and spent money in candidate elections, Our Revolution is required to comply with contribution limits, register with the FEC, and disclose its donors—but it hasn't," Paul S. Ryan, an attorney for Common Cause, told the Associated Press. "It's his establishment of the group that triggers these laws. That means a $5,000 limit, full donor disclosure and no contributions from prohibited sources."

Under FEC regulations, groups established by political candidates cannot "solicit, receive, direct, transfer, or spend funds" for federal election activity that exceeds legal limits. Individual contributions to a campaign are capped at $2,800 per election, while contributions to a political action committee are capped at $5,000.

The new complaint comes as Sanders has surged in national polls ahead of next month's Iowa caucus.

While Sanders has assailed big money donors and undisclosed cash in politics, Our Revolution has taken large checks while hiding its funding sources. The group has received nearly $1 million in contributions, including six-figure donations. It is working to increase voter turnout for Sanders and has spent money on social media advertisements.

Our Revolution did not respond to a request for comment.