The court will review a challenge by political science instructor William McConkey, who claims the referendum on the constitutional amendment illegally put two questions to voters: whether to ban gay marriage and whether to outlaw civil unions. The state constitution limits referendums to a single subject.
A Dane County judge upheld the referendum last year, and the Baileys Harbor man appealed that ruling. Last month, the Madison-based District 4 Court of Appeals asked the high court to take the case immediately because of its statewide significance.
Justices announced Thursday they decided to take the case and gave lawyers 30 days to file initial briefs. Oral arguments are not yet scheduled but are expected this fall, with a decision potentially before the end of the year.
Fair Wisconsin, the state's largest gay rights group, praised the court's decision to take the case.
"The constitutional amendment is definitely something we see as a stain on the constitution. It sort of enshrines discrimination," said its legislative director, Katie Belanger. "We are really looking forward to the Supreme Court making a fair decision about whether or not the amendment was put to the people in the legal and constitutional way."
A spokesman for Republican Attorney General J.B. Van Hollen pledged a vigorous legal defense of the amendment, which was approved by nearly 60 percent of voters.
"We have defended the voter's choice, and we will continue to do so," Bill Cosh said.
A ruling striking down the amendment would not legalize same-sex marriage because state law still defines marriage as a union between husband and wife. However, it could pave the way for lawmakers to eventually allow it, or for advocates to file lawsuits seeking that right.
Democratic Gov. Jim Doyle has already tried to chip away at the amendment's impact. Lawmakers are considering his plan to create a statewide domestic partner registry and give same-sex couples 43 benefits enjoyed by married couples, including the right to visit one another in the hospital and inherit each other's property.
The 2006 referendum asked whether to rewrite the state constitution to define marriage as between one man and one woman and outlaw the state from granting a similar legal status to unmarried individuals. The change was passed by lawmakers in two consecutive legislative sessions as required before going to the referendum.
The justices are expected to decide two issues. The first is whether the two-part question violated the clause in the constitution that limits referendum questions to a single subject.
The second is whether an individual voter such as McConkey, a straight man who has a gay daughter, has the legal standing to sue. Van Hollen argues he does not.
McConkey's lawyer, Lester Pines, said he was not surprised the high court decide to take the case since the law was unclear on both issues. He said the court had only three times interpreted the single-subject rule, most recently in 1984.
Pines said he was already looking forward to "a spirited and fun oral argument."
"This is a very significant legal issue," he said. "It's really not necessarily only about marriage. It's really about how things are supposed to be submitted to the voters."
In his ruling last year, Dane County Circuit Judge Richard Niess said McConkey could sue. But he ruled the two clauses in the referendum question were "two sides of the same coin" and properly decided with one vote.
"They clearly relate to the same subject matter and further the same purpose: the preservation and protection of the unique and historical status of traditional marriage," he said in a ruling from the bench.