The nation's most vociferous opponent of marriage equality on Monday announced that it would target Ohio Senator Rob Portman for defeat in 2016.

Portman last year became the first sitting GOP senator to endorse marriage equality.

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 that he had dropped his opposition to marriage equality after his son, Will Portman, told him and his wife Jane that he's gay.

Brian Brown, president of the National Organization for Marriage (NOM), said that he hopes that the defeat last week of three Republicans who support allowing gay couples to marry serves as a message to Republican leaders not to support such candidates in the next election cycle.

Two of the candidates, Carl DeMaio of California and Richard Tisei of Massachusetts, are openly gay. DeMaio's loss was most likely due to a last-minute scandal. The third candidate, Monica Wehby of Oregon, trailed behind incumbent Democratic Senator Jeff Merkley, who received nearly 56 percent of the vote.

(Related: Carl DeMaio says he's committed to more gay-inclusive GOP.)

However, it is doubtful that the candidates lost based on where they stand on the issue of marriage, because each of their opponents also supports marriage equality.

Nevertheless, Brown claimed credit: “I hope that our success in defeating these three Republican candidates sends a message to the Republican leadership in Washington that the GOP faithful demands candidates who are committed to defending marriage, which is a critical element of the Republican platform.”

Brown added that his group will oppose Portman's reelection or possible presidential bid.

“Rob Portman can forget about getting elected President of the United States,” Brown said. “If he runs we will make sure that GOP primary voters are aware of his desire to redefine marriage and his willingness to see federal judges set aside the votes of 50 million Americans who enacted marriage amendments across the country because his son is gay.”

“We intend to oppose Sen. Portman for reelection, and if he survives a primary challenge we will urge Republicans and Independents to refuse to vote for him in the General Election, just as we successfully did with DeMaio, Tisei and Wehby,” Brown said.