The Minnesota House on Thursday is expected to vote on a constitutional amendment that would ban gay marriage in the state.

Republicans with the exception of Representative Tim Kelly banded together at a Wednesday House panel to approve Republican Representative Steve Gottwalt's bill with a narrow 13 to 12 vote. A companion Senate measure has already cleared that chamber.

The House must approve the measure before Monday, the last day of the legislative session, for it to be on next year's ballot.

Minnesota law already bans gay and lesbian couples from marrying, but supporters of the amendment say the law remains vulnerable to legal challenges without the amendment.

The Minnesota Independent quotes Governor Mark Dayton as saying he'll fight the move “with every fiber of my being.”

The National Organization for Marriage (NOM), the nation's most vociferous opponent of gay marriage, took issue with gay marriage supporters' appeals to legislators.

“Gay marriage advocates are mounting a furious last-minute assault on Republican legislators in an effort to cow them into delaying the vote authorizing the marriage amendment until sometime next year,” NOM President Brian Brown wrote at the group's website. “They are rallying supporters all around the country to contact Minnesota legislators asking them to stop the marriage amendment.”

The Catholic Defense League of Minnesota sounded a similar alarm: “We have an opportunity, while investing minimal time, to fight back. As the legislators convene at the House chambers, there will be opposition forces there to try to intimidate them.”

According to a recent Star Tribune poll, a majority of voters (55%) oppose a constitutional amendment banning gay marriage, while 39 percent are in favor.