The Minnesota House will start debating at noon Thursday a measure that would legalize same sex-marriage, beginning what could be a historic turning point for gay and lesbian rights in Minnesota.

"This is one of those society-changing, breakthrough moments," said DFL Gov. Mark Dayton, a strong backer of the proposal. "The enormity of this bill cannot be overstated."

Opponents remain hopeful that their last-minute push can persuade a crucial few legislators to reject the change, pressing their belief that the DFL-controlled Legislature is going against the wishes of a majority of Minnesotans. They have spent months trying to convince lawmakers that marriage is a sacred union between one man and one woman ordained by God, not any state or even federal law.

"Traditional marriage supporters around the state are extremely upset that the DFL leadership, with same-sex marriage proponents, seem intent on forcing same-sex marriage on Minnesotans who don't want it," said Autumn Leva, a spokeswoman for Minnesota for Marriage, which is trying to defeat the measure. "They feel betrayed by their legislators."

In the closing days, same-sex opponents have pounded House Speaker Paul Thissen's office with calls, at the rate of one a minute. The Minneapolis Democrat said he would not bring up the measure for a vote unless he was certain it would pass.

The debate (which will be livestreamed here) stands to be the most divisive, dramatic and unpredictable at the Capitol this year. Advocates from both sides are expected to jam the Capitol in coming days to let their opinion be known and make sure legislators know they are watching.

Hundreds of gay and lesbian advocates are expected to pack the Capitol, wearing orange-and-blue "support marriage" stickers and shirts.

Minnesota for Marriage will run shuttles back and forth to the Capitol from the Cathedral and Living Word Church for its activists. They will wear Minnesota for Marriage blue-and-green and are planning a day of prayer, music and protest.

Capitol security will have an additional 40 officers on hand and four in the House gallery at all times. Many public areas will be roped off so lawmakers and staff can travel safely between rooms and exists. State workers in St. Paul have been told to steer clear of the Capitol unless they have specific work in the building.

Minnesotans United, the lead group pressing for same-sex marriage, has been reaching out to Republicans to give the measure a coalition of strong bipartisan support. Only one Republican – Sen. Branden Petersen of Andover – has announced he will vote to legalize same-sex marriage.

Nationally, hundreds of prominent Republicans have broken with the party orthodoxy on the issue and announced their support for same-sex marriage. Former Republican Gov. Tim Pawlenty spokesman, Brian McClung, recently announced his support for same-sex marriage.

If the House passes the measure, the Senate will vote on the proposal Monday. The Senate is widely believed to have a stronger majority in support of same-sex marriage.

Dayton is expected to sign the measure in a public ceremony next week.

Gay and lesbian couples could begin getting married in the state – or have their marriages from other states recognized – beginning Aug. 1.