MADISON, Wis. (AP) – Labor unions are joining forces to fight a proposed ban on same-sex marriage and civil unions in what could become a powerful force in the Nov. 7 referendum.

The groups, representing employees ranging from teachers to prison workers, say they are worried the amendment will take away their ability to bargain for benefits such as health insurance for the domestic partners of gay and straight employees.

They are making donations, organizing volunteers and educating their members as part of their attempts to make Wisconsin the first state to defeat a constitutional ban on same-sex marriage.

AFSCME, which represents 44,000 public service and health care workers in Wisconsin, became the latest to join the cause with a strong denunciation of the ban from its political arm and a vow to get its message out.

The unions are underscoring the main argument made by the ban’s critics: that it is not about same-sex marriage, which is already illegal in Wisconsin, but that it threatens a range of legal protections for all unmarried couples. Others say those fears are overblown.

Brian Weeks, director of AFSCME’s political arm, called the amendment “an attack on labor unions’ collective bargaining rights.” He said the group’s locals representing public employees for the city of Madison and Dane County stand to lose benefits if the amendment passes.

“Backers of this ban are trying to break deals and take away rights and protections that working people have earned through good-faith negotiations,” Weeks said.

That contention is hotly disputed by amendment supporters, who say gay rights activists are vastly distorting the potential impact to shift the terms of the debate.

The amendment would declare that marriage is between one man and one woman and that unmarried individuals cannot be granted a legal status similar to marriage such as civil unions pioneered by Vermont.

“It’s just inflammatory rhetoric,” said Julaine Appling, president of Vote Yes for Marriage. “This amendment isn’t going to change benefit structures that exist … It’s about whether or not we are going to live with a redefinition of marriage as something other than between a man and a woman.”

She also downplayed the significance of the unions’ positions, saying they do not speak for all of their members.

But Mike Tate, campaign manager of Fair Wisconsin, which is fighting the ban, said labor unions would help spread the group’s message that the amendment could have far-reaching effects.

“They are going to be focused on communicating to their members about why this is bad for Wisconsin, work with us on our door knocking and canvassing, and some have donated to the campaign already,” he said. “I think that labor unions are a really critical part of the coalition.”

The group’s latest campaign filing shows the Wisconsin Education Association Council, the state’s largest teachers’ union, gave $25,000 while AFT-Wisconsin, another public employees’ union, donated $5,000. The Milwaukee branch of Communication Workers of America gave $1,000.

Cathy Rought, spokesperson for 17,000-member AFT-Wisconsin, said the amendment threatens benefits enjoyed by some of technical college faculty and staff it represents. Other employees would not be able to bargain for similar benefits in the future, she said.

“We will be collaborating to go door to door and talk to people about the importance of defeating the amendment,” she said. “Most people, once they understand this is not an issue about being gay or being straight and that it’s an access to health care question, they oppose this.”