Gay marriage "is not the most front-burner issue," said Rep. James Lankford, R-Okla. "We still have the same legal issues we've always had with the Defense of Marriage Act," which he supports, he said.
Citing Obama's position, he said, "it's more a separation of powers issue than it is anything else." Lankford said other Republicans have supported same-sex marriage, so "this is not anything new."
White House spokesman Josh Earnest said Friday: "What is clear is that we are witnessing a pretty significant sociological shift in this country."
"It's happening right before our eyes in a way that says a lot about our country, that we have a country where we prioritize equality and fairness," he said.
Obama said last year he personally supports gay marriage, a step some liberals called overdue.
Polls show that public opinions on gay rights, including same-sex marriage, have shifted perhaps more rapidly than on any other major issue in recent times. In Gallup polling last November, 53 percent of adult Americans said same-sex marriages should be granted the same status as traditional marriages, while 46 percent felt they should not be valid.
Those figures were nearly reversed two years earlier. In 1996, when Gallup first asked about gay marriages, 27 percent felt they should be valid.
Many social and religious conservatives still oppose gay marriage. Some spoke up at the Conservative Political Action Conference, which was under way Friday in suburban Washington.
Randy Smith, a technology entrepreneur from California, said Portman's decision violates key conservative principles.
"Conservative values are based on God's word," Smith said. "If he is professing to be a Christian, I'd have no part of him."
Arne Owens, a "pro-family movement" activist from Virginia, said Portman's shift "does make it harder to maintain support for traditional marriage. There's no question about that."
John Radell, head of the Faith and Freedom Coalition of Delaware, said Portman's personal situation was difficult, "but that doesn't mean you stray from your faith."
"I understand loving your son," he said. "But Sen. Portman represents more than his son."
Radell said that "without question" Portman's shift would make his political future — particularly any presidential aspirations — more difficult.
Curt Steiner, an Ohio GOP consultant who helped run Portman's first House campaign in 1993, disagreed.
"I think it's always good to be forthright with your positions," Steiner said. "Some social conservatives will disagree with Rob Portman on this issue, but they understand the life that he leads, they understand his commitment to family, and they understand his commitment to them."
Steiner said that with the Supreme Court case nearing, more officeholders will be asked about gay marriage, and more Republicans may side with Portman.
The leader of a conservative group that promoted passage of a 2004 amendment in Ohio to ban gay marriage said he has heard from several people upset by Portman's stance.
"They feel betrayed," said Phil Burress, president of Citizens for Community Values. "They're not mad. They're sad and betrayed."
Ohio Republican Party Chairman Bob Bennett agreed that some Republicans are unhappy, but he said he received more phone calls Friday about the governor's budget. He said Portman has "taken a great deal of time to think it through and I certainly respect his right to make up his own mind."
Richard Socarides, who was President Bill Clinton's top adviser on gay issues, said Portman's son Will "proved once again that the most powerful political act any gay person can take is coming out." He said polls show that "people who know a gay person are far less likely to support discrimination."
Other prominent Republicans who have endorsed gay marriage include Sen. John McCain's wife, Cindy, and daughter, Meghan, as well as former first lady Laura Bush.
Associated Press writers Ken Thomas, Steve Peoples, Donna Cassata and Jim Kuhnhenn contributed to this report. Ann Sanner reported from Ohio.
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