Recommendations by the Maine Ethics Commission could force the National Organisation for Marriage to finally publish a list of its financial backers.
The anti-gay group has fought a long-running legal battle to keep its list of backers private.
The Maine Ethics Commission first launched an investigation into the group in 2009, after NOM refused to reveal fundraising and spending details for an anti-gay marriage ballot campaign in the state, thought to have cost over $2 million.
NOM has repeatedly gone to court over the issue, arguing that the Constitution allows protects people giving money anonymously, but the group lost successive rulings on the issue in 2009 and 2011, and had its appeal thrown out in 2012.
Maine Ethics Commission Executive Director Jonathan Wayne wrote: “After consideration of this evidence, there appears to be little doubt that NOM received far more than $5,000 for the purpose of promoting the Maine referendum.
“The staff view NOM’s failure to register and file financial reports as a significant violation of law. Maine people deserve to know who is funding political campaigns to influence their vote.”
According to the Bangor Daily News, the board of the Ethics Commission will meet on May 28 to discuss the final recommendations.
The Commission is recommending a $50,250 (£29,600) fine for the group, the largest ever recommended, and if the group do finally disclose their backers, it will likely have wider political ramifications.
Maine became the first state in the US to approve marriage equality by popular vote in 2012, when 53% of people in favour of same-sex marriage.