An Ohio group is calling on members to oppose Ohio Senator Rob Portman's reelection over his support for gay marriage.

Portman last week became the first GOP senator to publicly buck the party's official stance on the issue.

In an op-ed and later in interviews with CNN and the AP, Portman, a co-sponsor of the Defense of Marriage Act (DOMA), said he had dropped his opposition to marriage equality after his son, Will Portman, told him and his wife Jane that he's gay.

Citizens for Community Values, the group which led the 2004 campaign to approve Ohio's constitutional amendment that prevents the state from recognizing any relationship other than a heterosexual marriage, blasted Portman for telling the AP that response to his announcement has been mostly positive.

The Christian conservative group encouraged members to send Portman an email expressing disappointment with his new stance and vowing not to vote for him in 2016.

“I strongly disagree with your choice to support same-sex marriage,” the group's pre-written form email says. “I understand you've made a difficult family decision. But your personal situation won't change the principle of natural marriage overwhelmingly supported by the Republican Party.”

“You are now at odds with the National Republican Platform, the Ohio Republican Platform, Speaker of the House John Boehner, Ohio Governor John Kasich, as well as the majority of Republicans who elected you to represent them. Your support of same-sex marriage is a serious breach of the trust I had placed in you and I won't vote for you again.”