The National Organization for Marriage (NOM), national anti-gay marriage group, has been fined more than $50,000 (£30,000) for breaking Maine’s campaign finance laws.
The NOM was fined by the Maine Ethics Commission, which also ordered the group to file a campaign finance report.
In completing the report, NOM would reveal donors which supported a 2009 referendum which saw a majority of voters reverse the legislature’s same-sex marriage law before the measure came into force.
Lawyers acting on behalf of the organisation said it would file an appeal in Maine’s courts.
Political action committee Stand for Marriage Maine received almost $2 million (£1.2 million) from NOM.
Organisations are supposed to register as ballot question committees if they raise or spend over $5,000 (£3,000) to influence a state ballot question.
NOM says this was not necessary as none of the money was raised specifically for the cause.