President Obama’s endorsement of same-sex marriage is energizing Christian conservative support for Mitt Romney in a way that the likely GOP nominee has so far not been able to do on his own, according to religious leaders and activists.

Pastors in Ohio, North Carolina, Florida and other swing states are readying Sunday sermons inveighing against same-sex unions, while activist groups have begun laying plans for social media campaigns, leaflet drives and other get-out-the-vote efforts centered on the same-sex marriage issue. Romney could benefit from a strong turnout among evangelicals and other social conservatives, many of whom remain skeptical of his commitment to their causes. 

“So many people were rather lukewarm toward governor Romney and were really looking for some more tangible reasons to support him,” said Phil Burress, president of Citizens for Community Values, who led the ballot drive that banned gay marriage in Ohio in 2004. “Then lo and behold, it just fell out of the sky when Obama came out and endorsed same-sex marriage. . . . We are going to make this our key issue: the attack on marriage.”

The National Organization for Marriage, a leading anti-gay-marriage group, lashed out at Obama after his announcement and promised to campaign against him “ceaselessly” in swing states.

Same-sex marriage has long been a galvanizing political force for core constituencies in both parties, particularly conservative Catholics and evangelical Republicans. But the president’s public embrace of the idea alters the landscape in ways that promise to complicate the political calculus for both sides.

While more than 30 states now ban same-sex marriage and voters in North Carolina on Tuesday approved a constitutional amendment forbidding it, anti-gay-marriage activists acknowledge that public opinion on the issue has shifted dramatically since 2004, when ballot measures in Ohio and 10 other states helped drive social conservatives to the polls in support of George W. Bush.

Romney and other establishment Republicans have treaded softly on the issue so far, but many evangelicals think that a forceful anti-gay-marriage campaign could pay huge dividends for Republicans in the fall.

Some on the religious right also remain deeply uncertain about Romney’s convictions on cultural issues and are unhappy with his statements in recent days that he supports allowing gay couples to adopt children and that he does not view same-sex marriage as a religious issue. Many activists say they will continue to push Romney on the issue.

“Romney says he is for traditional marriage and then immediately says he is fine with homosexuals adopting children,” said David Lane, who organizes conservative pastors and congregations nationwide and helped lead anti-gay-marriage efforts in Iowa, California and other states. “Our base does not react well to that. They are not going to turn out [for a candidate] who tries to triangulate” on topics such as marriage and other traditional values.

The sentiments underscore the continued difficulties that Romney faces in attempting to navigate thorny cultural issues while attempting to woo independent voters with an economic message. Romney is slated to appear Saturday at Liberty University, an evangelical bastion in Lynchburg, Va., where he will deliver a commencement address. Romney says he is opposed to same-sex marriage and civil unions and favors a constitutional amendment banning gay marriages nationwide. But the former Massachusetts governor has also said that the “tender and sensitive” issue will not be a central part of his campaign.

Justin Peery, 27, a Virginia filmmaker who will attend Romney’s speech at Liberty, said he plans to vote for Romney this fall but is not passionate about his candidacy. That could change, he said, if Romney firmly denounced Obama’s endorsement of same-sex marriage.

“I don’t think it’s by coincidence that Obama came out in favor of gay marriage days before Romney will speak to a crowd of evangelicals,” Peery said. “If he talks about his opposition in a very clear way, he’ll probably get many, many standing ovations.”

Same-sex marriage does not appear to be the galvanizing force it once was, particularly among younger voters. A poll last year by the Public Religion Research Institute found that 44 percent of young white evangelicals favor allowing gay and lesbian people to marry, compared with 12 percent of evangelical seniors and 19 percent of evangelicals overall.

Some conservatives hope the issue drives a wedge between black voters, who largely oppose same-sex marriage, and the president. The Rev. John Coats II, who leads an African American church in Columbus, Ohio, said his Facebook page erupted on Wednesday with critical comments about the president from people who had previously defended him. He is already preparing to preach on Sunday posing the question: Why doesn’t the black community produce politicians who reflect the community’s values?

“I’m not saying they will be pushed to the point to vote for Romney,” Coats said of black voters. “But I believe it will increase voter apathy, and I have to say I’m surprised, and somewhat delightfully, that people are taking a closer look at this president who would never do that before.”

In Ohio, Burress said his group plans to re-create many of the tactics used in 2004 to spur social conservatives to the polls, including organizing rallies and meetings and placing tens of thousands of fliers in church bulletins across the state. The group will also renew its alliance with Amish voters there, who turned out in record numbers for the 2004 ballot measure, Burress said.

Aggressive use of Facebook, text messaging and other social media technologies will help amplify the message, he and others said.

“This could really be a difference-maker,” said J.C. Church, pastor of the 600-member Victory in Truth Ministries congregation in Bucyrus, Ohio, north of Columbus. “People will get fired up on this issue. North Carolina is an indication — this will awaken the conservative evangelicals to become aware, and the result will be action.”

But Robert Bennett, chairman of the Ohio Republican Party, said changing demographics and the struggling economy mean that same-sex marriage may not prove as central as it was eight years ago in the state. While he supports Romney’s position on the issue, he said the GOP presidential candidate would do best to focus on jobs and other economic issues.

“In Ohio, there are people on both sides of the issue who have largely made up their minds,” Bennett said. “Obviously it’s going to unite the social conservatives who maybe had some doubts about Romney, but there are other issues to unite people. This is more of a sidebar issue now.”

Staff writers Tom Hamburger in Washington and Krissah Thompson in Lynchburg, Va., contributed to this report.