WASHINGTON — The U.S. Supreme Court on Monday refused to hear an appeal of a Maine law that would require a national anti-gay-marriage group to name its contributors.

The decision clears the way for Maine officials to demand the group identify donors who helped bankroll the 2009 ballot campaign that overturned a state law allowing same-sex couples to marry. But the public release of any donor list could still be months away — if one is released at all.

The National Organization for Marriage has spent three years fighting to keep its donor list confidential. NOM was the largest donor in the 2009 campaign and is believed to be a major player in this year’s effort to defeat same-sex marriage rights in Maine.

Maine’s ethics commission is still investigating whether NOM is covered by the state disclosure law, an effort that has been hampered by several court cases, including a separate legal challenge by NOM in the Maine Supreme Judicial Court. In that case, NOM argues that the commission has limited authority to subpoena documents as part of its probe. No ruling is expected in that case till next year.

Maine’s campaign disclosure law requires groups spending more than $5,000 to influence the outcome of referendum elections to register as ballot question committees with the state ethics commission. NOM’s legal challenge to the federal high court claimed the law is too vague and inhibits free speech by discouraging donations.

Jonathan Wayne, executive director of the Maine ethics commission, called Monday’s decision “another affirmation that our campaign finance laws are constitutional.”

“The key for us is NOM should have to play by the same rules as everybody else,” said David Farmer, spokesman for Mainers United for Marriage, the group behind the effort to legalize same-sex marriage in November. “If they come into the state of Maine and spend millions of dollars, they should have to disclose the source of that money.”

NOM officials said they were disappointed with Monday’s decision. But the group also said Maine “substantially narrowed the type of information they were requesting” in the state’s filings with the Supreme Court.

“Had the state taken the position they took recently back in 2009, this litigation might well have been avoided,” John Eastman, NOM’s chairman said in a statement. “We will be reviewing the requests for information that the state has made in light of the narrow interpretation the state has now provided to its own statute.”

Assistant Maine Attorney General Phyllis Gardiner disagreed with Eastman’s statement.

“Our interpretation and the commission’s interpretation of the Maine statute has not changed during the course of this litigation,” Gardiner said.

NOM’s case against the ethics commission is one of several donor disclosure cases to reach the federal court system in recent years. Several observers who have followed these cases said they were not surprised that the high court sidestepped the Maine case.

“The Supreme Court decision not to get involved is further evidence that the court believes disclosure laws play a constitutionally vital role in our election system and, except in extreme cases, will be upheld,” said Adam Skaggs, senior counsel at the Brennan Center for Justice within New York University’s School of Law.

In their petition to the court, NOM’s attorneys had argued, among other things, that Maine’s “unconstitutionally vague and overbroad” disclosure law ran counter to the court’s decision in Citizens United.

Skaggs and others disagreed. While the Citizens United ruling did eliminate limits on independent expenditures and “super PACs,” it also upheld the constitutionality of state and federal disclosure laws, they said.

“The fact that the Supreme Court has repeatedly rejected those arguments has not stopped these groups from bringing case after case,” Skaggs said.

Following the 2010 Citizens United ruling, the Supreme Court heard a Washington State case in which gay-marriage opponents challenged a requirement that groups disclose the names of people who signed a referendum petition. The court upheld the Washington law.

So Bradley Smith, a former chairman of the Federal Election Commission and a vocal critic of campaign finance laws, was not surprised Monday that the court was not interested in taking up what he saw as “essentially the same case.”

Smith is a law professor at Capital University Law School in Ohio and co-founder of the Center for Competitive Politics, a conservative policy group that is strongly critical of campaign finance regulation.

Smith said concerns about loss of privacy when someone’s political contributions — and their address and employer — are disclosed are legitimate. He said political donors have been harassed in the past.

“Is it really so far-fetched when you see how nasty politics have gotten?” he asked.

NOM repeatedly cited donor harassment in its case against Maine. But Darrin Hurwitz, assistant general counsel to the Washington, D.C.-based Human Rights Campaign, a gay-rights advocacy group, said NOM and other groups haven’t backed up the claims.

“They have made essentially phony arguments of harassment of their donors,” Hurwitz said. “But the courts that have reviewed the evidence have found uniformly that any incidents that did occur were extremely isolated and were often perfectly legitimate forms of First Amendment expression.”

Shortly after the Supreme Court’s decision, NOM sent out a press release announcing the creation of a website, www.keeptherepublicandmarriage.com, where donors could “trumpet their support for NOM’s work in defense of traditional marriage.” NOM officials said the site caters to traditional-marriage supporters who were inspired by the CEO of Chik-Fil-A, who has publicly opposed same-sex marriage rights.

Washington Bureau Chief Kevin Miller can be contacted at 317-6256, or at:

kmiller@mainetoday.com