State campaign finance officials ruled Tuesday that those who give money to support or oppose ballot questions -- such as the next year's proposed amendment to ban same-sex marriage -- will have to disclose their donations.

The guidelines continue a months-long process of board action to define exactly what the law requires about money spent on constitutional campaigns and comes just as the fight over a constitutional amendment defining marriage comes to the fore.

"Minnesota has become ground zero for a national fight over whether the public should know who is behind political spending," said Mike Dean, executive director of Common Cause Minnesota, which pushes for more disclosure.

The board's policy would require donors' names to be made public if they specifically designated funds toward a ballot question or if they were asked by another organization to give money to support or oppose a ballot question. That could allow some donors to the National Organization for Marriage and other non-Minnesota-based organizations to go unnamed but would require disclosure of others.

Amendment supporter John Helmberger, the chairman of Minnesota for Marriage, said the campaign would comply with disclosure laws but that board's move amounted to "illegal regulations."

An attorney for the National Organization for Marriage, which opposes gay marriage, says the ruling may vastly increase the number of donors whose names would have to be revealed.

The tensions over donor disclosure are just a prelude to the fight that's just over the horizon in Minnesota. The constitutional question on whether to define marriage as the union of one man and one woman is expected to attract millions of dollars and a national spotlight to the 2012 vote.

The board's Tuesday decision may not be the final word on the issue. Conflicts over disclosure of marriage amendment campaign donors have landed in court in several states and the same may happen here.

Supporters of the 2012 proposed amendment disparaged Tuesday's decision.

"I think the board has muddied the waters dramatically," said Cleta Mitchell, attorney for the National Organization for Marriage. Mitchell said Minnesota is the first state in which a non-elected board extended the law in a way the Legislature has not.

Goldsmith said during the board meeting that several opponents of the guiding principles misunderstood them. The board, he said, acts to "limit rather than expand" statutes.

"This is not the making of new law," he said.

The National Organization for Marriage has acted aggressively in other parts of the country to protect donor names, bringing lawsuits to limit disclosure, according to Rick Hasen, a campaign finance expert and professor of law and political science at the University of California, Irvine.

"These lawsuits challenging disclosure rules have mostly failed," he said.

Mitchell said shielding donors' names is important because those who donate to support "traditional marriage" have been threatened and harassed.

After the conclusion of the board meeting, Chuck Darrell, communications director for Minnesota for Marriage, shared a flier for an Oct. 30 rally. The headline: "Marriage Equality Now!" In smaller print, the flier said "we will call out individual and corporate contributors standing in the way of equal rights."

"This is not speculative; this is real," Mitchell said.

Hasen said the U.S. Supreme Court has ruled that campaign disclosure laws must make "exemptions for people who face the threat of serious harassment."

Rachel E. Stassen-Berger • Twitter: @rachelsb