Republican state Sen. Branden Petersen is preparing to become a co-sponsor of a bill to legalize same-sex marriage in Minnesota.

Having a Republican co-author would be an enormous political coup for same-sex marriage advocates as they prepare to unveil their proposal in the days ahead. Petersen would become the first Republican legislator to publicly support same-sex marriage, highlighting the rapidly changing dynamics of the issue at the Capitol.

“At this point, I am concerned about doing the right thing,” said Petersen, an Andover resident who is married and has two young children. “I have a certain amount of peace about that, and I will let the chips fall where they may.”

Petersen was among a majority of Republican legislators who put a state constitutional amendment on the November 2012 ballot asking voters to add language banning same-sex marriage. Minnesota became the first state to defeat such an amendment after 30 others had passed similar measures; that result gave opponents what they say is significant momentum to return and try to erase the state’s long-standing law against same-sex marriage.

With passage far from certain, the fight is shaping up to be among the most divisive at the Capitol this year.

Republicans like Petersen are not the only ones who face the prospect of crossing lines against most of their party members on the issue. Several rural DFLers oppose same-sex marriage, bucking the party’s slim majority now in control of the Legislature.

“I feel strongly in my beliefs that it is not something I would support,” said Sen. LeRoy Stumpf, the only DFLer in the Senate who voted in favor of putting the marriage amendment on the 2012 ballot. “It’s a sacrament in our church. I’m Catholic.”

Stumpf, of Plummer, said that fellow DFLers understand his stance and did not pressure him to change his vote. He suspects others in the party will stand with him against same-sex marriage.

That means same-sex marriage advocates are likely to need a handful of Republicans to pass the measure.

“Republican support is something we want to make sure we have,” said Jake Loesch, a Republican and spokesman for Minnesotans United for All Families, the lead group trying to legalize same-sex marriage. “Republicans are weighing this issue. … As this conversation continues in the Legislature, there will be Republicans who will vote for marriage.”

Wants concerns addressed

Petersen said he has several concerns that must be addressed before he will sign onto the measure. He wants to add language guaranteeing that any religious leader can choose not to wed same-sex couples. He also insists that kids in same-sex marriages have the same financial guarantees as children of other married couples in time of divorce.

“It’s only a matter of time before same-sex marriage is legal,” Petersen said. “I thought it was important to engage the issue now, and when we do it, do it right, and that there’s some perspective from the people I represent in that.”

Sen. Scott Dibble, who is chief sponsor of the bill, said he is willing to agree to Petersen’s additions.

“Everything he has articulated, I see no problem with. At all,” said Dibble, DFL-Minneapolis.

He said he would be thrilled to have a GOP co-sponsor.

“It’s awesome,” he said. “I think it also allows a lot of space for other legislators to consider the same.”

Petersen said be believes his support will clear the way for other Republicans to join him in voting to legalize same-sex marriage.

“I know there are other Republicans who are very interested in supporting same-sex marriage,” he said.

Petersen, 27, admits this is a wrenching issue for him and could be politically damaging back home. His father-in-law has been in a same-sex relationship for nearly 20 years, but Petersen says this issue has fiercely divided his family in the same way it has split the rest of the state. He started discussing the issue with colleagues, his pastor and close friends before taking his public stance.

The first-term senator, who previously served as a House member, said he knows his position could cost him his seat. Residents in his district narrowly voted in favor of the marriage amendment in November. But as one of the most reliably conservative voters in the Legislature, Petersen said he believes voters in his district will find comfort in his record against taxes, his unwavering defense of the right to bear arms and his overriding belief in personal freedom.

“They are generally not single-issue voters,” he said. “But if push came to shove and that’s the way it had to be, then I am fine with that.”

Lobbying push is on

Legislators on both sides of the marriage issue are about to face enormous pressure on the issue. Lobbyists from both camps are racing between legislators’ offices, trying to see who is onboard with them and who is not.

Same-sex marriage supporters “spent a lot of time and a lot of money telling people that the constitutional amendment was unnecessary because there was actually a statutory law defining marriage,” said Autumn Leva, lobbyist for Minnesota for Marriage, which is trying to block same-sex marriage. “Now we see that same group is trying to push for a change in the law, and Minnesotans are very hesitant on that. They say, ‘Wait, wait, wait. I voted no on the amendment, but I didn’t vote to change the law.’ ”

Legislative leaders on both sides of the issue are doing a delicate political dance right now, neither lambasting the idea nor guaranteeing its passage.

DFL legislative leaders are not saying they plan to be crusading voices on the issue. Republicans who fought to get the marriage issue on the ballot and then lost their majorities are showing similar caution.

“If I had to vote in the next five minutes, I’d vote no,” said Sen. David Senjem, a Rochester Republican who is a former Senate majority leader. “I can’t imagine I would change my mind, but it’s a new day. You keep your options open, I guess.”

baird.helgeson@startribune.com 651-925-5044

rachel. stassen-berger@startribune.com Twitter: @rachelsb