ST. PAUL — Minnesota has a long and potentially polarizing campaign in store after state lawmakers agreed to allow voters to decide whether to limit civil marriage to heterosexual couples — a prohibition that already exists in state law.

The House voted 70-63 — District 27A Rep. Rich Murray voting against — just before midnight Saturday after nearly six hours of emotional debate that raised many of the issues likely to resonate in the coming campaign.

Critics of the amendment said it would divide families and neighbors and harm the dignity of gay people, while its supporters said the definition of marriage is important enough that voters alone — not judges or legislators — should decide how it’s addressed in the constitution.

The vote split mostly along party lines, with Republicans in support and Democrats opposed. Four Republicans crossed over to vote ‘no,’ and two Democrats voted ‘yes.’

The statewide vote itself is nearly a year and a half off, an unusually long lead time for both supporters and opponents to organize, raise money and try to sway Minnesotans. Both sides were already laying plans for respective coalitions for and against the amendment, with supporters calling themselves “Minnesota for Marriage” and opponents gathering under the banner of a group called “Minnesotans United for All Families.”

Gay rights activists said they believe public opinion about gay relationships is quickly shifting in their favor and that the next 18 months would give them time to reach a lot of voters with a message that gay relationships don’t threaten other families.

“This isn’t necessarily a case you can make to people in a sound bite,” said Monica Meyer, executive director of the gay rights group OutFront Minnesota. “But if you have the chance to sit down with someone and explain to them that this hurts real families and doesn’t help anyone, that’s how you win people over.”

Jason Adkins is executive director of the Minnesota Catholic Conference, which lobbied for the amendment at the Legislature. He said 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. He said they’d try to mount “the largest and most intensive grass-roots political campaign the state has ever seen.”

Adkins said their principle argument would be that “children thrive and are nurtured best when they are raised by a mother and a father.”

“Marriage under civil law is about attaching children to their parents, not just a relationship based on a consensual agreement between two adults who love each other,” he said.

Residents of 30 U.S. states have voted on the definition of marriage; opponents of gay marriage have prevailed in every single one of those votes. But recent polls have shown growing support for gay marriage, particularly among young voters, including a recent Star Tribune Minnesota Poll that found a majority of respondents opposed to the constitutional ban.

Few House Republicans spoke in favor of the ban in the Saturday night debate, a contrast to similar debates in past years. Democratic members showed no similar hesitation, and opposition was no longer limited to members from Minneapolis and St. Paul: Democrats from Bemidji, Brainerd, Hibbing, Duluth, Rochester, Mankato and the Twin Cities suburbs all took turns speaking against the amendment.

A few Republicans joined them. Rep. John Kriesel, a freshman Republican from Cottage Grove and an Iraq war veteran who lost his legs in combat, delivered an impassioned speech against the amendment.

“Happiness is so hard to find for people, so they find someone who makes them happy and we want to take that away?” Kriesel said. “We say you can be together but you can’t marry them? That’s wrong and I don’t agree with it.”

Rep. Rod Hamilton, R-Mountain Lake, said he supported giving voters the ultimate say. But he said his teenage daughter recently pointed out to him that she would be 18 by November 2012 — and that she would vote against it.

“She said, ‘Dad, I think a person should be able to marry whomever they love whether it’s the opposite sex or not,”’ Hamilton said.

Away from the Capitol, views on the amendment were complicated among voters in several suburbs south of Minneapolis.

Kathryn Cuhlmann, a 20-year-old massage therapist from Burnsville, said her religious upbringing influenced her belief that marriage should be reserved for opposite-sex couples.

“The Bible says marriage is between one man and one woman, I would vote to keep it that way,” Cuhlmann said. But she mentioned that finding out her closest friend, who recently died, was a lesbian had made her views evolve a bit.

“I’m not like ‘You guys can’t be happy,”’ Cuhlmann said. “I want them to be happy. So it’s kind of a hard spot to be in.”