For months, the proverbial “foreign hand” cast a shadow over Egypt’s transition to democracy. Now, the curse of the foreign passport has emerged as a potential spoiler in next month’s landmark presidential race.

According to Egyptian law, all presidential candidates — and their parents and spouses — must hold only Egyptian citizenship. The rule was included in constitutional amendments written by a military-appointed panel and approved in a national referendum last year. Many analysts described it as an attempt to bar former presidential candidate and revolutionary favorite Mohamed ElBaradei from running, because he once apparently held an Austrian passport.

But now the provision is haunting Hazem Abu Ismail, a politician running on an ultra-conservative, nationalist and anti-foreign intervention platform. On Saturday, the presidential election commission confirmed that Abu Ismail’s late mother became a naturalized U.S. citizen in 2006 and that it would notify the Islamic preacher of the finding.

The revelation effectively disqualifies Abu Ismail from the race and has caused an uproar among his followers, thousands of whom gathered Friday in Cairo’s Tahrir Square to protest what they called “manipulation.” On Saturday, they thronged a Cairo mosque where Abu Ismail forcefully denied that his mother had a U.S. passport. Earlier, he said in a statement that he had documents to prove his case and had hired American lawyers to help him.

Since the news broke, rumors have swirled of other candidates holding Qatari or Syrian nationality, spicing up a presidential race that has taken other unexpected twists and turns just before registration closes Sunday.

“It’s a ridiculous law,” said Khaled Fahmy, a historian at the American University in Cairo. “The real tragedy of this move is in a sense it strengthens the hand of the army.”

Fahmy said the military holds the power to decide on a whim who will run. On Friday, Omar Suleiman, who served as intelligence chief under former president Hosni Mubarak and is widely believed to be the military’s candidate, entered the race.

Late Saturday, the Muslim Brotherhood’s political wing announced plans to field a second, backup candidate. Mohammed Morsi, chairman of the Freedom and Justice Party, will also register, in case the ruling military council reverses its pardon of the Brotherhood’s main candidate, Khairat el-Shater, making him ineligible to run, the party’s statement said. Shater, the chief financier and strategist for the once-banned organization, was jailed several times under Mubarak, and a fellow presidential candidate filed a lawsuit questioning his eligibility, according to state media.

But Abu Ismail’s case is shaping up as one of the biggest controversies of the campaign. At the Ibn el-Furat mosque on Saturday, he blamed the United States for his likely exclusion and assured his supporters that the citizenship finding was a ploy to enable Mubarak-era figures such as Suleiman to return to power.

“This nationality issue, I assure you, is untrue,” he said. “What’s the Americans’ interest in issuing these documents at rocket speed?”

People climbed over fences to hear him. In the garden outside the mosque, vendors hawked T-shirts, pens and medals with his face on them. One woman wept as she said, “The Americans are excluding him intentionally.”

Special correspondents Ingy Hassieb and Haitham Mohamed contributed to this report.