CAIRO — An ultraconservative Islamist whose denunciations of American power have helped propel him to the front of Egypt’s presidential race appears to have been tripped up by his own American connections.

The mother of the candidate, Sheik Hazem Salah Abu Ismail, became an American citizen before she died, according to California public records and a Los Angeles voter registration Web site. That would disqualify Mr. Abu Ismail from running for president under current Egyptian law. And his exit would again scramble the race to become Egypt’s first president since the ouster of Hosni Mubarak and to potentially set the template for the government of a major Arab state.

Mr. Abu Ismail said his mother had obtained a green card granting her legal permanent residence, but not citizenship, and the incongruous image of an anti-American Islamist seemingly unaware of the details of his mother’s life in California is already delighting Egyptian liberals. And if Mr. Abu Ismail is forced to leave the race, American diplomats apprehensive about the possible repercussions of his victory may also be pleased. But in practical political terms his departure may help unite a fractured Islamist vote.

A spokesman for Mr. Abu Ismail’s campaign said it had sent a delegation to the United States to investigate. Presented with a report from a database of public records that included an address in Santa Monica, Calif., for his mother, Nawal Abel Aziz Nour, as well as her name on a Los Angeles voter registration list, the campaign spokesman, Mohamed Fahim Abdel Ghaffar, suggested it could be a forgery.

Interior Ministry officials, speaking on the condition of anonymity to discuss pending investigations, said separately on Wednesday that they had obtained copies of what they described as American “travel documents” belonging to Ms. Aziz Nour that indicated she had been a United States citizen before her death, but the exact nature of the documents could not be confirmed. A search of the Web indicated that Ms. Aziz Nour died within the last few years.

Mr. Abu Ismail’s campaign said Wednesday that he was filing a lawsuit in an administrative court here to force the Interior Ministry to publicly disclose its evidence.

“We’re going on with our campaign normally,” Mr. Abdel Ghaffar said. “Sheikh Hazem doesn’t want to make any comments to the media before he has made a strong legal response to these allegations.”

Mr. Abu Ismail has said that his sister, Hanan, had married an American and become an American citizen, and that his mother obtained a green card so that she could spend more time with her daughter.

In some public records, Ms. Aziz Nour and her daughter, Hanan Salah Abou Ismail, share a Santa Monica address. On Wednesday, a receptionist answered a phone number associated with Ms. Abou Ismail and confirmed that she was the sister of the Egyptian presidential candidate. Ms. Abou Ismail did not return phone calls seeking to clarify the situation.

A Los Angeles County Web site also confirmed that Nawal A. Nour, with same address and birthday as the Nawal A. Nour linked to Mr. Abu Ismail’s sister, had registered to vote.

The Egyptian election commission is expected to rule on Mr. Abu Ismail’s eligibility to run. Voting is scheduled to start in late May.

The most likely beneficiary of the controversy is Khairat el-Shater, the newly announced candidate of the Muslim Brotherhood. But backers of a more liberal Islamist, Dr. Abdel Moneim Aboul Fotouh, said they, too, hoped to take Mr. Abu Ismail’s supporters. Although culturally more liberal than Mr. Abu Ismail, Mr. Aboul Fotouh has struck similar populist notes on economic issues, emphasizing government support for the poor in contrast to the Brotherhood’s business-friendly emphasis on free markets.

Before Mr. Mubarak was ousted last spring, the Constitution stipulated that only candidates whose parents were both Egyptian citizens were eligible for the presidency, although those rules remained hypothetical since Egypt never held a free presidential election.

But when the generals who seized power after Mr. Mubarak left office issued a “constitutional declaration” for the interim government, they bowed to a surge in national pride and also barred candidates with a parent who held citizenship in any other country even if both parents were also citizens of Egypt.

At the time, the rule was considered most likely to work against liberal candidates friendly to the West. Voters drawn to Mr. Abu Ismail’s brand of Islamic populism were all for it.

As speculation about Mr. Abu Ismail’s mother heated up this week, though, those Western friendly liberals began to breathe sighs of relief. Just days ago, many of them were trading nervous jokes about the recent appearance on seemingly every wall in Cairo of posters bearing Mr. Abu Ismail’s bushy, white beard and broad, toothy smile.

By Wednesday, though, the liberal banter had turned to taunts. Some suggested that Mr. Abu Ismail was either lying or short-sighted, or perhaps did not know his own mother’s passport. Others compared him to the ultraconservative Islamist who became Egypt’s first post- Mubarak political scandal casualty. That politician had sought sympathy for what he said was a vicious beating sustained during an armed robbery, but his bandages turned out to cover a nose job.

Sheelagh McNeill contributed reporting from New York, and Mayy El Sheikh from Cairo. (Copyright 2012 The New York Times Company 04/05/12)