Sen. Rob PortmanRobert (Rob) Jones PortmanTax rules will be subject to more OMB review under new memo Ending sex trafficking tomorrow requires preventing child abuse today Doctors bristle at push for opioid prescription limits MORE (R-Ohio) has suffered a slight loss in support from Republicans after becoming the first GOP senator to back gay marriage, according to a new poll.

The poll, conducted by Quinnipiac University, shows a four-percentage-point decline in Ohioans who approve of the job he's doing, down to 40 percent approving and 31 percent disapproving.

That's a shift from 44 percent approving and 24 percent disapproving in a late-February survey, taken before his mid-March announcement that he had changed his mind on gay marriage after learning his son is gay.

Forty-one percent of Republicans polled said they think less favorably of Portman following his shift, and the percentage of respondents reporting an unfavorable view of the senator saw a sharp increase in the new poll, up to 21 percent from 8 percent in late February. His approval among Republicans has dropped six percentage points since the last poll.

A majority of Ohioans polled, however, say Portman's support for gay marriage has made no difference on their perception of him, while 20 percent think more favorably and 25 percent think less favorably of him.

Overall, Ohioans remain split over gay marriage, with slightly more, 48 percent, saying they support it, than the 44 percent who do not. But that's an increase in support from December, when another Quinnipiac poll showed a plurality, 47 percent, opposed it, indicating Ohio is following the same trend shown in most national polls on the issue.

That means though Portman is currently suffering a loss in support, the issue likely won't have a lasting impact on his electoral prospects in 2016, when he's up for reelection. 

Portman is also considered by some to be a potential presidential contender, and his stance on gay marriage could be a boon in a national campaign, as Republicans look to try to expand the party's appeal to untapped voting blocs.

Following Portman's announcement of his support for gay marriage, Sen. Mark KirkMark Steven KirkHigh stakes as Trump heads to Hill Five things to watch for at Trump-Senate GOP meeting Giffords, Scalise highlight party differences on guns MORE of Illinois also came out in support. But the issue is still contentious within the Republican Party, as evidenced by the Republican National Committee (RNC) reaffirming its opposition to gay marriage at an RNC meeting last week.

The Quinnipiac poll was conducted among 1,138 registered voters from April 10-15 and has a margin of error of plus or minus 2.9 percentage points.