ST. PAUL — A gay Minnesota lawmaker leading the push to legalize same-sex weddings in the state told colleagues Thursday that gay couples “contribute to the same Minnesota system as everyone else” and deserve the right to be married.

The Minnesota House was expected to cast what was viewed as a pivotal vote that would position the state to become the 12th in the country to allow gay marriages and the first in the Midwest to pass such a law out of its Legislature.

Debate in the House got underway in the early afternoon, as thousands of supporters and opponents gathered outside the chamber. House Democratic leaders and the bill’s supporters were confident it would pass. If it passes Monday in the Senate, Democratic Gov. Mark Dayton could be sign it into law by early next week and make Minnesota the 12th U.S. state to legalize gay marriage.

“My family knew firsthand that same sex couples pay our taxes, we vote, we serve in the military, we take care of our kids and our elders and we run businesses in Minnesota,” said the bill’s sponsor, Rep. Karen Clark, a Minneapolis Democrat who is gay. “… Same-sex couples should be treated fairly under the law, including the freedom to marry the person we love.”

Opponents argued the bill would alter a centuries-old conception of marriage and leave those people opposed for religious reasons tarred as bigots.

“We’re not. We’re not,” said Rep. Kelby Woodard, R-Belle Plaine. “These are people with deeply held beliefs, including myself.”

In the debate’s first hour, House members adopted a Republican-offered amendment to strengthen religious protections that backers hoped would draw a few GOP votes for the final bill. The House also rejected a Republican amendment to swap marriage for civil unions.

The Minnesota push for gay marriage grew out of last fall’s successful campaign to defeat a constitutional amendment that would have banned it. Minnesota became the first state to turn back such an amendment after more than two dozen states had passed one over more than a decade.

Eleven other states allow gay marriages — including Rhode Island and Delaware, which approved laws in the past week. Minnesota would be the first state in the Midwest to pass the measure out of the Legislature. Iowa allows gay marriages because of a 2009 court ruling.

Leaders in Illinois — the only Midwestern state other than Minnesota with a Democratic-led statehouse — say that state is close to having the votes to approve a law too. But most other states surrounding Minnesota have constitutional bans against same-sex weddings, so the change might not spread to the nation’s heartland nearly as quickly as it has on the coasts and in New England.

Demonstrators chanting and waving signs choked the Capitol’s marbled hallways leading up to the debate, and passion was evident on both sides as gay marriage supporters waved bright orange signs and opponents hoisted pink ones. The crowds prompted heighted security at the Capitol.

Among the demonstrators was Grace McBride, 27, a nurse from St. Paul. She said she and her partner felt compelled to be there to watch history unfold. She said she hopes to get married “as soon as I can” if the bill becomes law. The legislation would allow her to do so starting Aug. 1.

“I have thought about my wedding since I was a little girl,” she said.

On the other side of the divide, the Rev. Steve Goold of New Hope Church led followers in a morning prayer before they set out to lobby lawmakers. He told them they had the power to change minds, but urged them to be respectful.

“Do not shout and boo. Pray,” Goold said. Galina Komar, a recent Ukrainian immigrant who lives in Bloomington, brought her four-year-old daughter and one-year-old son to the Capitol to express her religious concerns.

“I do believe in God, and I believe God already created the perfect way to have a family,” Komar said.

But gay marriage supporters also boasted faith leaders in their ranks.

“I’ve celebrated marriages for same-sex couples, but I’ve never been able to sign a marriage license for any of them,” said the Rev. Jay Carlson, pastor at a Minneapolis Lutheran church. “I look forward to the day when I can.”

Democrats hold 73 of the 134 House seats, but at least two from more-conservative rural districts intended to oppose the bill. Some others have said they wouldn’t discuss their vote prior to casting it.

Rep. Jason Isaacson, a Democrat from the St. Paul suburb of Shoreview, recalled how when he was a student, classmates were mocked with homophobic names.

“In middle school that was one of the worst things you could call somebody else,” Isaacson said. “I like to think today that is changing.”