For years, it's been a staple of the White House calendar.

On the first Thursday of May, dedicated as the National Day of Prayer, President George W. Bush hosted an ecumenical service in the East Room, a big public endorsement of evangelical Christians. (This event is different from the National Prayer Breakfast, held outside the White House gates every year on the first Thursday of February.)

President Obama opted not to have a service in the White House this year.

"Prayer is something that the president does every day," explained White House press secretary Robert Gibbs, adding that Obama will sign a proclamation to recognize the day. "I think the president understands, in his own life and in his family's life, the role that prayer plays."

President Truman signed the first National Prayer Day proclamation, and President Reagan made it a permanent occasion. Under Bush, the day was a political event, confirming a conviction that religion was a core tenet of Republican politics.

Naturally, prominent evangelicals, including National Day of Prayer Task Force Chairman Shirley Dobson, and her husband, Focus on the Family founder James Dobson, expressed disappointment in Obama's decision.

"At this time in our country's history, we would hope our president would recognize more fully the importance of prayer," Shirley Dobson said in a statement.

But Obama has shown an unusual sensitivity toward atheists, the first president to mention non-believers in an inaugural address when he said, "For we know that our patchwork heritage is a strength, not a weakness. We are a nation of Christians and Muslims, Jews and Hindus and non-believers."

Non-believers took heart at Obama's decision but urged him to go further.

"It's not his job to tell people to pray," David Silverman, national spokesman for the organization American Atheists, told CNN. "We are very happy he did away with the George W. Bush-era celebrations and party, but we wish he wouldn't do it at all."

-- Johanna Neuman

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