Ohio Senator Rob Portman, the first Republican senator to endorse gay marriage, has lost GOP support in the month since his announcement.

According to a Quinnipiac poll released Friday, Portman's approval rating dipped slightly from 44 percent to 40 and disapproval of his job performance increased from 24 percent to 31.

Disapproval among Republicans increased 13 points. Forty-one percent of GOP voters said they view Portman less favorably because of his reversal on the issue.

Support for marriage equality increased slightly among Ohioans since the institute's December survey, from 45 perecent to 48. Forty-four percent remain opposed.

“Sen. Rob Portman's reversal on same-sex marriage has cost him a little support in his Republican base, but has little impact among Democrats and independent voters,” said Peter Brown, assistant director of the Quinipiac University Polling Institute.

“The movement in his numbers has not been massive and the overall movement towards acceptance of same-sex marriage, which we see in Ohio and elsewhere, could help him in the long run, especially if he seeks reelection four years from now.”