Amid chants of "We've only begun to fight," about 300 same-sex marriage supporters marched to Loring Park in Minneapolis on Sunday, signaling the unofficial start of a 17-month campaign they hope will make Minnesota the first state in the country to block a constitutional amendment that would forbid gay marriage.

The march and rally came barely 12 hours after the Minnesota House, in an emotion-packed, hours-long debate, voted 70-62 to put an amendment on next year's ballot that would define marriage as the union of a man and woman.

They predict a nasty, expensive campaign that could be second only to President Obama's re-election bid as the dominant issue of the 2012 political season.

Ann Rubin of Minneapolis, who attended the rally with Sandy Almquist, her partner of 11 years, is braced for the fight.

"I'm afraid and scared, and it's going to be divisive," Rubin said. "For the next 17 months, we have to concentrate on letting people know us and see us. We're just like everybody else. We're not frightening. We're not scary. We work, pay bills and walk our dogs, and we just want a chance to participate with the same legal rights."

Tom Prichard, president of the Minnesota Family Council, which successfully lobbied to put the issue in front of voters, said Sunday that "our goal is to not make it personal. I think we can have a respectful discussion and conversation on the importance of marriage in our state, where there's widespread support that the best environment to raise children is with a loving mother and father."

Prichard predicted that Minnesotans will follow 31 other states who have defined marriage in their constitutions. "The people should decide, not the courts or the Legislature, and fortunately this vote allows that to happen," he said.

State Rep. Karen Clark, DFL-Minneapolis, told rally-goers that "we will not be 32." She was joined by another Minneapolis DFLer, Sen. Scott Dibble, who told the crowd, "We've been punched in the gut, but we're a strong, resilient people."

Clark and Dibble are the only openly gay members of the Legislature.

Other states have seen deep divisions in their amendment battles.

In Michigan alone, the Catholic Church raised most of the $1.2 million to pass a 2004 amendment defining traditional marriage in that state's constitution.

In Maine, the debate was peppered with late-campaign charges that a "yes" vote would mean teaching homosexual sex in public schools to third-graders, which gay rights activists decried as fear-mongering lies.

The Minnesota marriage amendment battle will include the influx of national groups and money. On Monday, the National Organization for Marriage (NOM) weighed in with a statement, promising that it will be “supporting the campaign and lending our expertise and resources.

“The vote Saturday night represents another string of stringing defeats for the gay-marriage group Human Rights Campaign and their state allies,” NOM President Brian Brown said, “and is further concrete evidence that the widely-reported claim that same-sex marriage is inevitable is a lie.”

Gearing up on the Web

To no one's surprise, the marriage amendment's first battleground appears to be online.

An "I Plan to Vote NO to the MN Marriage Amendment on the November 2012 Ballot" Facebook page already attracted more than 21,500 "likes." Within 15 minutes of the bill's passage at about midnight, a website was launched by a group of amendment opponents called Minnesotans United For All Families.

The House vote did not break strictly along party lines. Two DFLers supported the ballot question and some Republicans voted "no."

"I just thought it was not appropriate to have that as a constitutional amendment on the ballot," said Mound Rep. Steve Smith, one of four Republicans to vote against the amendment. He said about 70 percent of his constituents are against the amendment. "I don't always run with my pack."

Rep. Denise Dittrich, DFL-Champlin, voted for the amendment, saying the debate "was a distraction to our job, which is to balance the state budget" and should be put to voters. She said she expected "an informed and respectful debate."

For an amendment to pass, a majority of those voting need to mark “yes.” If a voter skips the questions, that amounts to a “no” vote. The actual ballot language will be: “Only a union of one man and one woman shall be valid or recognized as a marriage in Minnesota.“

Gov. Mark Dayton, who opposes the amendment, has no power to veto a constitutional amendment legislators approve for the ballot.

Minneapolis Mayor R.T. Rybak called the amendment a "job killer" that will discourage businesses from growing in Minnesota. He urged the Loring Park crowd to take the fight beyond their computers, announcing a two-front strategy called "Work Out" and "Go Home" that would have gay rights advocates make their case at their workplaces and their hometowns, reaching back even to childhood friends and football teammates.

Recent surveys that rank Minneapolis as the "gayest" city in the country could spawn complacency, some gay rights supporters fear. A recent Star Tribune poll indicated that 55 percent of respondents oppose the amendment, while 39 percent favor the traditional marriage definition. Polls in California and Maine showed amendment opponents leading right up to the vote, which broke the other way.

"People have to get it out of their heads that, because we're in Minnesota, it will fail," said Philip Lowe of Richfield. "That 'there's no way we can lose' is exactly how things started in California and Maine."

Barbara Mahowald, a retired teacher and principal from Savage, decided Sunday morning to come out with a handmade cardboard sign that said, simply: "Taking Back Minnesota." Mahowald said she worries about the message the coming debate will send to kids and is concerned about a possible increase in bullying and suicides.

"This is the start of Wisconsin in Minnesota,'' Mahowald said. "I've been thinking of coming to a protest for some time, and the vote last night was the last straw. If our state doesn't wake up, we'll be having the same problems as Wisconsin."

Clark, who gave an emotional floor speech before the amendment was passed, said on Sunday that she is trying to stay positive. "Right now, I don't know. I'm trying to keep positive energy, but I'm very angry underneath."

Shortly after it passed, the Minnesota Catholic Conference praised lawmakers for putting the amendment question on the 2012 ballot.

"The voters of Minnesota, like those in ... other states, have been given the opportunity to have an important conversation about the future of marriage in this state," said Jason Adkins, executive director of the Minnesota Catholic Conference, which supported the amendment. "We look forward to a respectful and civil conversation among Minnesotans about why the amendment should be adopted. We are grateful to the Legislature for putting this amendment on the ballot."

Staff writer Rachel E. Stassen-Berger contributed to this report. Curt Brown • 612-673-4767