PORTLAND — A national group that opposes gay marriage is appealing a federal judge’s decision to uphold Maine’s campaign finance disclosure law that could force it to reveal its list of donors.

Lawyers for the National Organization for Marriage filed their notice of appeal with the 1st U.S. Circuit Court of Appeals in Boston, which already is considering another constitutional challenge of Maine’s campaign finance law by the Virginia-based organization.

The National Organization for Marriage, or NOM, sued after the state ethics commission found that it could be required to disclose the identities of its donors.

Last month, U.S. District Judge D. Brock Hornby granted a summary judgment upholding Maine’s campaign disclosure laws as they pertain to ballot committees.

Maine law says groups that raise or spend more than $5,000 to influence elections must register with the state and disclose their donors. NOM donated $1.9 million to Stand for Marriage Maine, a political action committee that helped repeal Maine’s same sex marriage law.

The Maine Commission on Governmental Ethics and Election Practices still must conduct an investigation to determine whether NOM must turn over its donor list, the disclosure of which NOM feels would stymie free speech, said Josiah Neeley, an Indiana-based lawyer for NOM.

“Our view is even the investigation, or the possibility that these requirements would be enforced, creates a chilling effect on the First Amendment rights, not only of the National Organization for Marriage, but of other political groups that might want to get involved in a ballot campaign,” Neeley said.

Jonathan Wayne, executive director of the ethics commission, said he was pleased that the state’s disclosure laws pertaining to ballot committees stood up to scrutiny. “It lets the public know who’s influencing them when the vote on ballot questions,” he said.

Already pending before the Boston appeals court is a separate appeal brought by NOM of election law governing political action committees, independent campaign contributions and other attribution and disclaimer elements required by Maine’s election laws.

In that case, Hornby upheld most of Maine’s reporting regulations applying to political action committees. But he agreed with NOM that a state regulation requiring 24-hour disclosure of any independent expenditures over $250 is unconstitutionally burdensome.

NOM opposes same-sex marriage and has been involved in campaigns in Maine and other states. Maine voters repealed the state’s same-sex marriage law in 2009.