Brian Brown, the president of the National Organization for Marriage (NOM), on Tuesday predicted election defeats for Senators Rob Portman of Ohio and Mark Kirk of Illinois over their support for gay marriage.

During an appearance on the Janet Mefferd Show, Brown told radio host Janet Mefferd that embracing marriage equality “would be the death knell for the Republican Party.”

“Right now the Democratic Party has changed its platform, has wholeheartedly embraced the redefinition of marriage. … We need to be encouraging Republican lawmakers to be speaking out more on the importance of marriage, not attempting to imitate the Democratic Party in embracing the redefinition of marriage.”

Brown said that Portman and Kirk might decide to retire from the Senate in 2016 to avoid facing Republican voters in a primary.

“They need to be primaried, period. I think that folks in Ohio, if Rob Portman decides to run again, he will be primaried. He may not run again because there's been such a backlash in his state, and I think the same is true of Mark Kirk,” said Brown.

According to a recent survey, Portman's approval rating among Ohioans dipped slightly from 44 percent to 40 percent after he announced his support for marriage equality. However, disapproval among Republicans increased 13 points and 41 percent said they view Portman less favorably because of his reversal on the issue.

(Related: Rob Portman loses GOP support over gay marriage endorsement.)