But his “new beginning” has aroused nervousness in some, especially those who disagree with his counterterrorism policies, or those more comfortable with a vision of America as a white and largely Christian nation, and not the pluralistic melting pot Mr. Obama represents.

The debate over the proposed Islamic center in Manhattan only intensified on Saturday, as the conservative blogosphere lighted up with criticism of Mr. Obama, and leading Republicans — including Newt Gingrich, the former House speaker; Representative John A. Boehner, the House minority leader; and Representative Peter T. King of New York — forcefully rejected the president’s stance.

Mr. Gingrich accused the president of “pandering to radical Islam.” Mr. Boehner said the decision to build a mosque so close to ground zero was “deeply troubling, as is the president’s decision to endorse it.” And Mr. King flatly said the president “is wrong,” adding that Mr. Obama had “caved in to political correctness.”

Indeed, the criticism was so intense that the White House ultimately issued an elaboration on the president’s clarification, insisting that the president was “not backing off in any way” from the comments he made Friday night.

“As a citizen, and as president,” Mr. Obama said then, “I believe that Muslims have the same right to practice their religion as everyone else in this country. And that includes the right to build a place of worship and a community center on private property in Lower Manhattan, in accordance with local laws and ordinances.”

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The local issue of the mosque and the wider issues of Islam and religious freedom are just part of a divisive cultural and political debate that is percolating in various forms during this hotly contested election season. On Capitol Hill, for instance, some Republicans advocate amending the Constitution to bar babies born to illegal immigrants from becoming citizens — a move the president also opposes.

“I think it’s very important, as difficult as some of these issues are, that we stay focused on who we are as a people and what our values are all about,” the president said here on Saturday.

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Mr. Obama’s predecessor, George W. Bush, also held annual Ramadan celebrations and frequently took pains to draw a distinction between Al Qaeda and Islam, as Mr. Obama did Friday night. But Mr. Obama, unlike Mr. Bush, has been accused of being a closet Muslim (he is Christian) and faced attacks from the right that he is soft on terrorists.

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“For people who already fear the worst from Obama, this only confirms their fears,” said John Feehery, a Republican consultant who spent years as a top party aide on Capitol Hill. “This is not a unifying decision on his part; he chose a side. I understand why he did this, but politically I think it’s a blunder.”

White House aides say Mr. Obama was well aware of the risks. “He understands the politics of it,” David Axelrod, his senior adviser, said in an interview.

Few national Democrats rushed to Mr. Obama’s defense; party leaders, who would much prefer Mr. Obama to talk about jobs, were mostly silent. Two New York Democrats, Senator Kirsten E. Gillibrand and Representative Jerrold Nadler, however, did back Mr. Obama. But Alex Sink, the Democratic candidate for governor here, distanced herself, while Gov. Charlie Crist, a Republican-turned-independent, defended the president.

“I think he’s right,” Mr. Crist told reporters during an appearance with the president at a Coast Guard station here.

Mr. Obama has typically weighed in on such delicate matters only when circumstances have forced his hand, as he did during his campaign for president, when he gave a lengthy speech on race in America in response to controversy swirling around his relationship with his fiery former pastor, Jeremiah Wright.

Debate about the Islamic center had been brewing for weeks, yet Mr. Obama had studiously sidestepped it.

But the Ramadan dinner seemed to leave the president little choice. Aides said there was never any question about what he would say.

“He felt that he had a responsibility to speak,” Mr. Axelrod said.