Franklin Graham -- son and legatee of Billy Graham, the traditional spiritual advisor to the nation’s presidents -- is lashing out at President Obama for his support of gay marriage, saying that Obama has “shaken his fist” at God, and lamenting “a sad day for America.”

Graham, the president and chief executive of the Billy Graham Evangelistic Assn., criticized the president in a statement Thursday, a day after the president told ABC News that his personal belief was that “same-sex couples should be able to get married.”

Graham’s three-paragraph statement, released on the association’s website, noted that voters in his home state of North Carolina had approved a constitutional amendment that allows only heterosexual marriage. Overall, 38 states have adopted some kind of prohibition on gay marriage, and the president said that the states should be able to determine whether to accept homosexual unions.

There may be more at stake than the relationship between Obama and a family of preachers who have counseled presidents going back to Harry S. Truman. Polls show that a slight majority of Americans support gay marriage, but how the president’s new position will play out in swing states, including North Carolina, is far from clear.

If social issues begin to define the election, they could distract presumptive Republican nominee Mitt Romney from his main message that Obama has been a poor steward of the economy. But an issue like gay marriage could also fire up a conservative evangelical base on Romney’s behalf, superseding concerns about his Mormon faith.

As a senator and presidential candidate, Obama had said that he opposed gay marriage, but more recently said that his position was evolving.

“In changing his position from that of “Senator/candidate Obama, President Obama has, in my view, shaken his fist at the same God who created and defined marriage,” Graham said. “It grieves me that our president would now affirm same-sex marriage, though I believe it grieves God even more.

“The institution of marriage should not be defined by presidents or polls, governors or the media,” Graham continued. “The definition was set long ago and changing legislation or policy will never change God’s definition. This is a sad day for America. God help us.”

Graham, 59, has insinuated previously that he’s not a big fan of the president. On Feb. 21, on MSNBC’s “Morning Joe” show, he was asked whether Obama was a Christian.

“I think you have to ask President Obama,” he said. Because Obama’s father was a Muslim, Graham added, “under Islamic law, the Muslim world sees Barack Obama as a Muslim.”

Elsewhere, Graham has called Islam “wicked and evil.”

Billy Graham, 93, has retired from public preaching, but he joined his son in publicly urging North Carolina voters to support Amendment One, which outlawed gay marriage in the state.

“Watching the moral decline of our country causes me great concern,” the elder Graham said in newspaper advertisements published around the state. “I believe the home and marriage is the foundation of our society and must be protected.”

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richard.fausset@latimes.com