Hardline US Republican Senator Rob Portman, who last month reversed his opposition to equal marriage, has lost some support, mainly from members of his own party, a survey has found.

The survey, by Quinnipiac University, found that Senator Portman’s approval rating is currently 40% among Ohio voters, which had dropped four points from a poll on 28 February.

His disapproval rating also went up from 25% in February to 31%.

The poll indicated that a six-point drop in approval by self-identifying Republicans, was mainly the reason for his drop in popularity. Just more than 40% of Republicans said they favoured the Senator less for his change of stance on equal marriage.

In February, 63% of Republican respondents said they did approve of Senator Portman’s efforts as Senator, which had dropped slightly to 57%.

Senator Portman who was among the original sponsors of the Defense of Marriage Act (DOMA) became the first Republican Senator to announce that he has changed his anti-equal marriage stance. It followed the personal revelation of his own son’s coming out as gay.

An adviser to Senator Portman responded to the poll, saying that his change of stance on the issue had nothing to do with politics.

“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 the adviser.

Peter Brown, assistant director of the Quinnipiac University Polling Institute, said: “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.”

The poll also revealed that 48% of Ohio voters were in support of legalising equal marriage, and 44% were against.

President Obama’s approval rating in the state had also dropped from 54% in December, 48% in March and down to 45% now.

The Quinnipiac University survey was conducted April 10-15, with 1,138 registered voters questioned by phone. The overall sampling error was plus or minus 2.9 percentage points.

The dip in Senator Portman’s support was within the survey’s sampling margin of error.

Senator Portman spoke at a Republican Party late in March, and said that the audience was “very respectful” of his change in stance 

He quickly drew criticism from religious groups who previously supported him, as Phil Burress of the Ohio group Citizens for Community Values, spoke out to say that he thought Portman was “a very troubled man”, and suggested that he was “distraught” over his son coming out.

In an interview, the United States House Speaker John Boehner said that he “can’t imagine” he would “ever” change from his current stance in opposition to equal marriage.

Last weekend, the US Republican Party’s national committee voted unanimously to reaffirm its opposition to equal marriage, and to urge the Supreme Court to uphold Proposition 8, California’s ban on same-sex marriage, and the Defense of Marriage Act.