Nearly one in five people, or 18 percent, said they think Mr. Obama is Muslim, up from the 11 percent who said so in March 2009, according to a poll released Thursday. The proportion who correctly say he is a Christian is down to just 34 percent.
The largest share of people, 43 percent, said they don't know his religion, an increase from the 34 percent who said that in early 2009.
The survey, conducted by the nonpartisan Pew Research Center and its affiliated Pew Forum on Religion & Public Life, is based on interviews conducted before the controversy over whether Muslims should be permitted to construct a mosque near the World Trade Center site. Mr. Obama has said he believes Muslims have the right to build an Islamic center there, though he's also said he won't take a position on whether they should actually build it.
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In a separate poll by Time magazine/ABT SRBI conducted Monday and Tuesday - after Mr. Obama's comments about the mosque - 24 percent said they think he is Muslim, 47 percent said they think he is Christian and 24 percent didn't know or didn't respond.
In addition, 61 percent opposed building the Muslim center near the Trade Center site and 26 percent said they favor it.
The Pew poll found that about three in 10 of Mr. Obama's fiercest political rivals, Republicans and conservatives, say he is a Muslim. That is up significantly from last year and far higher than the share of Democrats and liberals who say so. But even among his supporters, the number saying he is a Christian has fallen since 2009, with just 43 percent of blacks and 46 percent of Democrats saying he is Christian.
Among independents, 18 percent say Mr. Obama is Muslim - up from 10 percent last year.
Pew analysts attribute the findings to attacks by his opponents and Mr. Obama's limited attendance at religious services, particularly in contrast with President George W. Bush and President Clinton, whose worship was more public.
Andrew Kohut, the Pew Research Center's director, said the confusion partly reflects "the intensification of negative views about Obama among his critics." Alan Cooperman, the Pew Forum's associate director for research, said that with the public hearing little about Mr. Obama's religion, "maybe there's more possibility for other people to make suggestions that the president is this or he's really that or he's really a Muslim."
Mr. Obama is the Christian son of a Kenyan Muslim father and a Kansas mother. From age 6 to 10, Mr. Obama lived in predominantly Muslim Indonesia with his mother and Indonesian stepfather. His full name, Barack Hussein Obama, sounds Muslim to many.
White House officials did not provide on-the-record comments on the survey, but they prompted Pastor Kirbyjon Caldwell of Houston to call The Associated Press.
Caldwell, who said he has known Mr. Obama for years, said the president is a Christian who prays every day. He said he was not sure where the public confusion about the president's religion came from, but he called false media reports about it "a 24-hour noise box committed to presenting the president in a false light."
Six in 10 of those saying Mr. Obama is a Muslim said they got the information from the media, with the largest portion - 16 percent - saying it was on television. Eleven percent said they learned it from Mr. Obama's behavior and words.
Despite the confusion about Mr. Obama's religion, there is noteworthy support for how he uses it to make decisions. Nearly half, or 48 percent, said he relies on his religion the right amount when making policy choices, 21 percent said he uses it too little and 11 percent too much.
At the same time, the poll provides broad indications that the public feels religion is playing a diminished role in politics today, with fewer people than in 2008 saying the Democratic and Republican parties are friendly toward religion.
With elections for control of Congress just over two months away, the poll contains optimistic news for Republicans. Half of white non-Hispanic Catholics, plus three in 10 unaffiliated with a religion and a third of Jews support the GOP - all an increase since 2008.
The survey also found:
• The Democratic Party is seen as friendly to religion by 26 percent, while 43 percent say the same about the GOP. That's a 9 percentage point drop for Republicans since 2008, and 12 points lower for Democrats.
• Fifty-two percent say churches should stay away from politics, a reversal of the slim majorities that supported churches' political involvement from 1996 to 2006.
The poll, overseen by Princeton Survey Research Associates International, involved landline and cell phone interviews with 3,003 randomly chosen adults. It was conducted July 21-Aug. 5 and has a margin of sampling error of plus or minus 2.5 percentage points.