Brian Brown, president of the National Organization for Marriage (NOM), has criticized Alaska Senator Lisa Murkowski's endorsement of gay marriage.

On Wednesday, Murkowski became the third Republican senator to make such an endorsement, saying that allowing gay couples to marry was consistent with her values as a “lifelong Republican” who believes in “promoting freedom and limiting the reach of government.”

“Senator Murkowski has sealed her political fate,” Brown said in a blog post. “Alaskans voted by an overwhelming majority (68%-32%) to protect marriage in their Constitution when given the opportunity, and an even stronger majority of Republicans in Alaska supported that move; thus, her betrayal of marriage is tantamount to political suicide.”

Brown added that Murkowski had “fallen for the lies of same-sex 'marriage' activists” and threatened a campaign against her reelection.

“NOM will be active in educating Alaskan voters about Senator Murkowski's choice to betray marriage and disrespect the convictions of a majority of her constituents.”

Alaska was one of the first states to pursue a constitutional amendment limiting marriage to heterosexual couples. Nearly 15 years have passed since voters approved Ballot Measure 2 in 1998. According to statistician Nate Silver, Alaskan voters would reject such an initiative today.

Ohio Senator Rob Portman was the first GOP senator to make such an endorsement. Portman said in March that the issue had become personal for him after his son came out gay. Roughly two weeks later, Illinois Senator Mark Kirk followed suit, saying that after suffering a major stroke he realized that “our time on Earth is limited.”