Minnesota voters will get to decide if their state constitution defines marriage as the union of a man and a woman.
The state house voted late Saturday night 70-63 after nearly six hours of emotional debate to put the definition of marriage on the ballot in November 2012.
The vote was split along party lines, with Republicans in support of the measure and Democrats opposed. The state senate approved the vote last week.
State law already prohibits gay marriage, but supporters of the proposed amendment say it's necessary to prevent judges or lawmakers from legalizing it in the future. They said Minnesotans should decide how it's addressed in the constitution.
Jason Adkins, executive director of the Minnesota Catholic Conference, helped lobby for the amendment at the Legislature.
He told the Associated Press if opponents of the amendment think they can win over the public, then they wouldn't have lobbied the Legislature so hard to reject it.
"If they were so confident of winning the debate in the public square, I'm not sure why they were so afraid to put it on the ballot," Adkins said.
He said the Minnesota for Marriage group would be a joint venture of Catholic and evangelical churches and other religious and secular groups.
Adkins said they'd try to mount "the largest and most intensive grassroots political campaign the state has ever seen."