(CNN) - The approval rating for the first Republican senator to back same-sex marriage has edged down, fueled mostly by a drop in support from members of his own party, according to a new survey.
An adviser to Sen. Rob Portman of Ohio, responding to the poll, says the senator's change of heart on the controversial social issue had nothing to do with politics.
A Quinnipiac University survey released Friday indicates that Portman's approval rating stands at 40% among Ohio voters, down four points from a February 28 poll. The four percentage point dip is within the survey's sampling error. The senator's disapproval rating went from 24% in February to 31% now.
A six point drop in approval by self-described Republicans appears to be the main factor in the overall edging down of Portman's approval. Sixty-three percent of Republicans questioned in the February poll said the approved of the job Portman was doing as senator. That's dropped to 57% now. And just over four in ten Republicans say they think less favorably of Portman because of his switch on same-sex marriage.
In March, Portman became the first Republican in the U.S. Senate to endorse same-sex marriage. He announced that he was swayed on the issue by his son, who told his family he was gay.
"Rob's change of heart was driven by a family issue, and clearly had nothing to do with politics or poll numbers. He remains focused on the top issues in the minds of Ohioans – our economy, and Washington's out of control spending and debt," said a Portman adviser.
Portman's change of stance came just before the U.S. Supreme Court heard two high profile cases involving same-sex marriage. The high court is expected to issue rulings in June.
Portman, a former congressman who served as U.S. trade representative and director of the Office of Management and Budget under President George W. Bush, was first elected to the Senate in 2010 and is not up for re-election until 2016.
"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 Quinnipiac University Polling Institute.
According to the poll, 48% of Ohio voters support legal same-sex marriage, with 44% opposed.
The survey also indicates President Barack Obama's approval rating in Ohio now stands at 45%, down from 48% in March and 54% in December.
The Quinnipiac University poll was conducted April 10-15, with 1,138 registered Buckeye State voters questioned by telephone. The survey's overall sampling error is plus or minus 2.9 percentage points.
CNN Political Editor Paul Steinhauser contributed to this report