The increasingly costly and bitter fight over a constitutional amendment to ban same-sex marriage is a statistical dead heat, according to a new Star Tribune Minnesota Poll.

Six weeks before Election Day, slightly more Minnesotans favor the amendment than oppose it, but that support also falls just short of the 50 percent needed to pass the measure.

Among likely voters, 49 percent would approve constitutional language that defines marriage as only the union of a man and woman. Another 47 percent oppose the measure, while 4 percent are undecided. Minnesota law requires any change to the Constitution to capture a majority of all ballots cast. That means a voter who skips the question is counted as a no vote.

While passions for and against a traditional definition of marriage run strong, the poll also shows that attitudes are shifting when it comes to civil unions.

The poll shows overwhelming support for civil unions that would offer gay and lesbian couples the same legal rights as marriage. Overall, 68 percent of respondents would support civil unions while only 23 percent would oppose them. The support cuts across every demographic and party line. The poll interviewed 800 likely voters between Sept. 17 and 19 and has a margin of error of plus or minus 3.5 percentage points.

Deadlock on the marriage amendment offers the freshest proof yet that the fight stands to be among the closest, most expensive and least predictable of the election season.

Both sides said they are buoyed by the poll's results.

The slight edge for the pro-amendment side, while not outside the margin of error, shows "we are in good shape and reinforces our belief that if we execute our game plan, we will pass the amendment," said Frank Schubert, who leads that effort for Minnesota for Marriage. The other side, he said, "has not moved the needle" from a year ago, even after a months-long campaign.

But opponents, who launched two television advertisements in the last week, said the poll shows they are well-positioned to defeat the measure.

"It's clearly an incredibly close race and will be a dead heat down to Election Day," said Richard Carlbom, campaign manager for Minnesotans United for All Families, the lead group opposing the measure.

The demographic divides

The poll reveals a significant split among voters, depending on age and where they live.

Hennepin and Ramsey counties are strongholds of opposition, with 58 percent saying they would vote no. Head to the suburbs outside those two counties and the numbers flip, with 59 percent saying they would support the measure.

Minnesota's oldest voters were the most likely to approve the amendment, with 55 percent of those 65 and older favoring the amendment. Younger voters are just as opposed, with 57 percent saying they will vote against it.

After months of furious back and forth on the issue, few Minnesotans are without an opinion. In nearly every category undecideds are in the low single digits.

The gender and political leanings of respondents reveal further divides.

Fully 57 percent of men say they support the measure, but 56 percent of women were opposed.

Party affiliation is another strong fault line, with 78 percent of Republicans in favor and 73 percent of Democrats opposed. Independents, often crucial swing voters in any election, are coming down on the side of the amendment by 55 percent to 38 percent opposed. About 7 percent were undecided -- the single-largest block of undecided voters in the sample on the marriage question.

Reasons for their stands

Minnesota law forbids same-sex marriage, but amendment supporters say the change is needed to prevent judges or legislators from changing the law later to legalize same-sex marriage.

"Marriage has been marriage for many thousands of years," said Norbert Timm, a 78-year-old retiree from Minnetonka. "To change it now to make it into something else, you'd have to be a radical idiot."

But Timm said he is completely comfortable with civil unions. "Civil unions are just another legal thing, that is not trying to change people's religion or the character of the world," he said.

Several respondents said their faith led them to their decision on the marriage amendment.

"I am pretty much for marriage between a man and a woman," said Mark Bingham, a 55-year-old farmer from Fosston who has a gay son. "I go to the Catholic church and that's what they tell us, and that's what I believe."

Amendment opponents argue that it would be the first-time voters enshrined discrimination in the Constitution.

"Anybody who wants to get married should be able to," said Lorna Gardner, a 76-year-old retiree from Duluth.

She called the amendment "mean-spirited," and believes someday soon, gay marriage will be legal. "The other side can make as much noise as they can, but this is going to happen."

Many marriage amendment opponents are not satisfied with civil unions, which they say relegate gay couples to second-class status. They vowed to press on for full rights.

"When it comes to marriage, there's nobody who grows up dreaming of going to the courthouse to get a civil union," Carlbom said. "There's nothing more stabilizing than two people who are in love and want to make a commitment to one another. A civil union just isn't that."

Baird Helgeson • 651-925-5044