Ohio Senator Rob Portman, the first Senate Republican to endorse same-sex marriage, was arguably the one who opened the floodgates for Democratic senators to announce their support for marriage equality. In the weeks following his op-ed in the Columbus Dispatch, one Democratic senator after another announced support for same-sex marriage, solidifying the consensus for marriage equality that has emerged in the Democratic Party.

Republicans, on the other hand, haven’t been as willing to take that step. Only one other GOP senator has come out in support of same-sex marriage — Mark Kirk of Illinois — and just last week, the Republican National Committee voted to affirm its opposition to marriage equality, satisfying the social conservatives who have threatened to bolt the party if it changes its position. And while Portman was applauded by gay rights advocates for his change of heart, his constituents aren’t so happy with his decision to embrace same-sex marriage.

According to the latest poll from Quinnipiac University, Portman’s approval has dipped somewhat since endorsing marriage equality. Last month, 44 percent approved of his work and 24 percent disapproved. Since then, his disapproval rating has risen to 31 percent. And it’s entirely do to Republican discontent with his stance. Forty-one percent of self-identified Republicans in the state say they view Portman less favorably because of his shift. Overall, 20 percent say it reflects poorly on Portman, 25 percent say it reflects positively, and 53 percent are indifferent.

None of this is a big surprise — only 48 percent of Ohio voters support same-sex marriage, compared to the slight majority support that exists nationwide. But it does explain why Republicans — who may privately favor marriage equality — aren’t going to announce their support anytime soon. Social conservatives are still an important group within the Republican Party. They provide donors and volunteers, and are a key constituency in GOP nomination contests at all levels.

Explicit support for same-sex marriage might appeal to the general public, but it alienates a large portion of the Republican base. Rob Portman can’t take back his endorsement, but it’s almost certainly true that other lawmakers have noticed Portman’s declining popularity, and will act accordingly.

Jamelle Bouie is a staff writer at The American Prospect, where he writes a blog.