Khairat al-Shater, who is also the Islamist group's chief strategist and financier, said that vote fraud might lead to the election of a "new Mubarak" who would spark a "new revolution."
His remarks show how Egypt's presidential election is turning into a battle between the country's new rising political power – the Islamists – and the old institutions of power, including the military and the intelligence services, from Mubarak's day.
El-Shater's "new Mubarak" refers to the former president's spy chief Omar Suleiman, who announced his nomination on Friday. The Brotherhood as well as liberals and secularists fear he may be propelled by powerful forces in the military and security services into the presidency.
Suleiman's candidacy is "reinventing a new Mubarak regime, with a new look," el-Shater said.
Both sides have suggested that the race is turning ugly, with dirty tricks and worse.
The entry into the race by Mubarak's most trusted deputy and intelligence chief Suleiman has sparked Islamists' concern that the generals are trying to undercut the Brotherhood's power in order to protect their privileges.
Suleiman was one of the biggest foes of the group, which was banned and faced intermittent crackdowns under Mubarak. Last year, Suleiman warned that Islamists are behind revolutions in the Arab world.
"He can't win unless there is fraud," el-Shater said of Suleiman. "I don't want fraud and I don't hope to see fraud but everything is possible and if it happened there will be a new revolution."
Like many in the Brotherhoo, el-Shater spoke mostly in broad strokes about what the movement would do if it could: build a nation with "Islam as a reference."
The Brotherhood has tried to allay the fears of Christians and secularists who fear that it might restrict minority or women's rights or other freedoms, although some of its critics say that it is only making a show of moderation.
The Brotherhood is also under fire from liberals and others who claim that it is using its parliamentary majority to dominate the process of writing a new constitution.
But the Brotherhood argues that it has a mandate from the voters.
"The people chose us," el-Shater said, "They are waiting for us to solve their problems and to carry forward their renaissance ... It's not manly or courageous that we let them down."
But he suggested that the Brotherhood might be blocked from implementing its agenda by other powers.
"We don't know if presidential elections will be completed or not, we don't know if after elections we will have a government that expresses the majority or not, whether a constitution is going to be written," he said.
"Ambiguity clouds the whole situation and there is a struggle over power."
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