The fight over allowing same-sex marriage last year could claim its first political casualty as a GOP legislator faces an insurgent challenger from his own party over his support for the legalization effort.

Rep. David FitzSimmons, R-Albert­ville, is immersed in a bitter and increasingly personal battle with newcomer Eric Lucero, a technology manager and Dayton City Council member who has made FitzSimmons’ support for same-sex marriage the centerpiece of his campaign.

The defining battle will come Saturday, when GOP activists gather to endorse a candidate for the House seat. FitzSimmons says he will abide by the party’s endorsement, creating a make-or-break moment for the first-term legislator.

“Losing is a distinct possibility,” he said earlier this week. “I think I have a solid shot to win, but anything can happen.”

FitzSimmons was among just four House Republicans who broke with their party to vote for legalization last spring. Of those four, one is not seeking re-election. The others have raised significant money and so far withstood challenges from within.

Ousting FitzSimmons would be a strong statement from the conservative flank of the GOP and hand a rare win to opponents of same-sex marriage after a string of high-profile defeats in Minnesota.

“It’s kind of sad when we are in this place, where constituents have to act to find someone who better represents their values,” said Autumn Leva, a spokeswoman for the Minnesota Family Council, which is working to defeat FitzSimmons.

Lucero, who repeatedly turned down requests for interviews, has framed himself as a crusader for traditional family values. His website features a picture of him with a lion in the movement, U.S. Rep. Michele Bachmann, R-Minn.

In an earlier version of Lucero’s website, he had a special section listing “assaults against the family.” It included “gay marriage and the homosexual lifestyle,” allowing women to serve in combat, and allowing gays and lesbians to serve openly in the military.

Lucero said in a statement to the Star Tribune this week that residents “do not trust David FitzSimmons to accurately represent them.”

Lucero’s website features a section called “David FitzSimmons breaks his word,” and highlights a letter in which the legislator assures a constituent he will not support changing the marriage law.

“At the time, voters had every reason to trust David FitzSimmons,” Lucero says on the website. “David FitzSimmons later broke his word by voting to redefine marriage.”

FitzSimmons said he agonized over his decision leading up to the vote but was persuaded by changes made to protect churches that don’t want to perform same-sex weddings. Since then, a growing number of states have embraced same-sex marriage, and courts around the nation are tossing out bans in state constitutions.

‘I stand by it’

FitzSimmons said he has no regrets. “I had the opportunity to protect churches from a law that was coming,” he said. “Knowing what I know and seeing the direction it is going nationally, I am more convinced now that it was the best possible outcome. And I stand by it.”

Lucero and FitzSimmons live in one of Minnesota’s most conservative districts. In the past election, 53 percent of voters in that district supported a failed constitutional ban on same-sex marriage.

His vote aside, FitzSimmons is among the most conservative members in the Legislature, and he hopes his reliable opposition to abortion, taxes and new spending will persuade constituents of his fealty to Republican principles.

In a show of loyalty, FitzSimmons gave about $26,000 from his campaign to a larger GOP effort to win back control of the House and to other GOP groups in his district. That left his campaign with just $15.16. The political action group for Minnesotans United for All Families, the group that pushed for same-sex marriage, sent FitzSimmons’ campaign a check for $1,000. He never cashed it.

He is a longtime supporter of Bachmann, who is not seeking re-election, and of former legislator Tom Emmer, who is the front-runner in the race to replace her in the Sixth Congressional District.

Getting nasty and personal

The FitzSimmons-Lucero fight has turned nasty and personal just in the past few days.

Sen. Mary Kiffmeyer, a Big Lake Republican who represents FitzSimmons’ district, has not rushed to defend her fellow legislator.

“The thing people hate the most is when you campaign on one thing and then do another,” Kiffmeyer said. “It annoys them a lot, and I got a lot of phone calls from people who were really annoyed with that. It makes people feel discouraged.”

More recently, critics have raised questions about FitzSimmons’ romantic relationship with Sarah Walker, a Capitol lobbyist who served as a board member for the group that pushed to legalize same-sex marriage.

Hanover Mayor Chris Kauffman sent a letter to supporters this week, saying they are in a “heated battle for conservative values” and calling FitzSimmons a social liberal.

“Do you think it is a conflict of interest if legislators are dating lobbyists?” he asked in the letter, noting FitzSimmons’ ties to Walker.

The personal attacks caused friends to rush to the defense of FitzSimmons, a single father who never hid his relationship with Walker around the Capitol.

The personal attacks are “way over the line,” Emmer said. “People can disagree on a vote, but they cannot question David FitzSimmons’ character or integrity.”

Adding to the drama, the political arm of Voices of Conservative Women on Thursday made the unusual move of sending out a letter in support of FitzSimmons. It is the first time the group, committed to limited government and fiscal responsibly, has endorsed a man.

These next couple of days could become a turning point in FitzSimmons’ political future.

“I voted against the tax bill. I voted against the spending bills. I am a huge backer of Tom Emmer,” FitzSimmons said. “I have yet to hear any other record, or any vote, or anywhere where anyone tried to say I am not a conservative.”

On Saturday, Lucero will be the first. “Politics of the modern day,” he said in his statement, “is ripe with feelings of distrust by people toward politicians.”