A substantial and growing chunk of the country believes that President Obama, a self-described Christian, is Muslim, while only about a third of Americans are able to correctly identify his religion, according to a survey released Thursday.

Nearly one in five Americans believe Obama is a Muslim, up from around one in 10 Americans who said he was Muslim last year, according to the survey, conducted by the Pew Forum on Religion and Public Life.

While most of those who think Obama is Muslim are Republicans, the number of independents who believe he is Muslim has expanded significantly, from 10 percent last year to 18 percent now.

The number of Americans who express uncertainly about the president's religion, meanwhile, is much larger and has also grown, including among Obama's political base. For instance, fewer than half of Democrats and African-Americans now say that Obama is Christian.

In March 2009, 36 percent of African-Americans said they didn't know what religion Obama practices. Now, 46 percent of African-Americans say they don't know.

"You would think the longer the person is in the White House, the more the 'don't knows' would decline," said Alan Cooperman, the Pew Forum's associate director for research. "But the 'don't knows' are higher now than when he came to office."

The survey was conducted in late July and early August.

Though Obama advertised his Christianity on the campaign trail and early in his administration - including distributing pamphlets about his religion during the 2008 presidential race and inviting the Rev. Rick Warren to his inauguration - he has been less public about his faith since then.

Despite intense media speculation about which Washington church Obama would join, for example, the White House has yet to announce that he has joined any.

"We had eight years of George W. Bush, who was very public about religious debates and high profile about religious practice and that's followed by Barack Obama, who is much lower profile about religious beliefs and practices," Cooperman said.

"It could be that in the relative vacuum of information coming out of the White House about his personal religious beliefs, others step in to feel the breach," Cooperman said. "It allows others who say that 'Oh, he's really this or that' to gain some currency."

Joshua DuBois, executive director of the White House Office of Faith-Based and Neighborhood Partnerships, said Wednesday night that Obama has "expanded in a historic way the engagement of persons of faith by this administration."

The president has given six speeches on faith issues, DuBois said, and has launched the first-ever White House advisory council for the faith-based office, composed largely of religious leaders.

"A lot of these facts are not necessarily what the public and the media are focused on everyday, which is not surprising given the issues we're facing as a country: reforming health care, bringing the troops home from Iraq and the economic recovery," DuBois said in an interview.

False rumors that Obama is Muslim have dogged him since he declared his candidacy for president in 2007. Pew conducted its survey before the president's comments last week about the right of Muslims to proceed with a controversial proposal for an Islamic center and mosque near ground zero.

The Rev. Joel Hunter, a Florida evangelical who is in frequent touch with Obama, says their relationship belies the findings of the new survey.

"He is very definitely a Christian, but a lot of the things he does to work on spiritual formation are simply not public," Hunter said.

Hunter said that he is in weekly contact with the president about his spiritual life, including writing devotionals for Obama and praying with him via telephone. Hunter said he received a call from Air Force One on the president's 49th birthday earlier this month.

"Several of us (Christian pastors) prayed with him over the phone," Hunter said. "We talked about his life and what he wanted us to pray for and it was at his initiative."

Earlier, when the president learned Hunter's grandchild had been stricken with cancer, the Florida preacher said he received a call from the White House.

"He called and told me that he and Michelle were praying for us," Hunter said, referring to the first lady. "I explained that this was an aggressive form of cancer and he pastored me, saying the Lord would be with us through this and that we should trust in God. It was a real reversal of roles."

But Hunter said the administration may want to reconsider its messaging on religion in light of the Pew poll.

"It may be time for them (the White House) to be a little more public about what the president does to be an active Christian," he said.