Obama, who became a churchgoing Christian as an adult, has tried to reach across this religious gap. He is the first president to directly recognize the growing number of secular Americans in an inaugural speech.
"We are a nation of Christians and Muslims, Jews and Hindus, and nonbelievers," Obama said in 2009.
Americans with no religious affiliation generally vote Democratic, and are expected to become as important to Democrats as evangelicals are to Republicans. A Pew analysis found 10 members of the new Congress have no stated religious affiliation and one, Arizona Rep. Kyrsten Sinema, says she specifically has no religion. All are Democrats.
Obama has also developed friendships with several evangelical leaders, including Giglio, who visited the White House and prayed at Obama's Easter prayer breakfast last year. But those efforts have sometimes upset the president's more liberal constituents.
At the 2009 inaugural, an uproar followed his choice of prominent pastor Rick Warren to give the invocation. Like Giglio, Warren hadn't made gay marriage a central focus of his ministry. But in an interview around the time of his selection, Warren had compared gay relationships to incest and pedophilia. Despite pressure from gay rights advocates to back out, Warren gave the invocation.
The tradition of including clergy prayers at inaugurals has followed a complex path, with a gap spanning more than a century.
George Washington attended a chapel service as part of his official swearing-in, but the tradition didn't stick. Although worship services were often held surrounding the inauguration, prayer at the inaugural ceremony itself was abandoned until 1937, when President Franklin D. Roosevelt reinstated the practice.
Roosevelt never unequivocally said why. But at the time, the popular Roman Catholic radio priest, the Rev. Charles Coughlin, had been attacking the president, arguing the New Deal helped banks, not American citizens. Another prominent priest, the Rev. John Ryan, publicly defended Roosevelt and the president asked him to offer an inaugural prayer, apparently as thanks for his loyalty, according to Martin Medhurst, a Baylor University political scientist who has researched inaugural prayer.
Roosevelt balanced the ticket religiously by including a Protestant clergyman to reflect the American religious majority.
Over the next few decades, presidents chose a combination of clergy from a cross-section of major American religions. Along with a Protestant and Roman Catholic, Harry Truman added a rabbi. A Greek Orthodox churchman was later featured. In 1969, Richard Nixon included a leader of the historically black African Methodist Episcopal Zion Church.
But just as the podium had grown crowded with clerics, the presidents-elect began to scale back. And since 1989, at the inaugural of George H.W. Bush, the honor has been bestowed exclusively on Protestants. The Rev. Billy Graham, known as "America's pastor," was the most frequent celebrant.
On Inauguration Day, another clear sign of the times will be found in a hotel away from official events. "The Presidential Inaugural Prayer Breakfast" will feature the Rev. Pat Robertson, Minnesota Rep. Michele Bachmann, Missouri Sen. Roy Blunt and others. The keynote speaker is expected to discuss the prophetic destiny of America.
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