The two top Islamists running in Egypt's first real presidential race share a common history.
Abdel Moneim Aboul Fotouh, a physician, is a former senior leader in the Muslim Brotherhood whose moderate stance has made him popular not only with Islamists, but with liberal and secular Egyptians.
Mohammed Morsi, an engineer, heads the Brotherhood's political party, which holds nearly half the seats in parliament.
Yet despite their common political background, the two men are bitter rivals.
The competition between them could also split the vote among Islamists, and both of them trail Amr Moussa, the former secretary general of the Arab League, in the run-up to voting that starts on May 23.
When Aboul Fotouh decided to run for president, he broke party discipline and the Muslim Brotherhood kicked him out, says Nathan Brown, a professor of political science and international affairs at George Washington University.
"The division between him and the rest of the movement is bitter and personal. It's not simply about ideas," he says. "It's really about someone who they feel betrayed the cause and betrayed the organization which he was supposed to be upholding."
That enmity has garnered unlikely backers for the moderate Aboul Fotouh. Among them are hard-line Islamists known as Salafists, whose Nour Party controls about a quarter of the seats in parliament.
Nour Party spokesman Nader Bakkar defends the choice. He says they aren't asking Aboul Fotouh to be a cleric, but a head of state. He adds that Morsi, the Brotherhood candidate, just doesn't measure up to the job.
Brown, the George Washington professor, says the endorsement reflects longstanding mistrust the hard-liners have for the Brotherhood, which they view as more political than religious.
"They've been rivals quietly on Egyptian streets and in Egyptian villages and towns for a while, and now they are political rivals as well," Brown says. "They have some common elements in their platforms, but they are very acutely aware that to the extent that either one of them has a challenge, it's probably more from the other, say, than it is from seculars or liberals who are just a small number in the society."
But the Brotherhood is playing down any rivalry.
The group's spokesman, Mahmoud Ghozlan, says the Salafists are free to endorse whom they want. He adds the Brotherhood is prepared to work with whoever becomes president.
Aboul Fotouh's campaign adviser, Rabab al-Mahdi, also dismissed the idea of any animosity toward the Brotherhood.
"And now that he's presenting himself as a president for all Egyptians, [Aboul Fotouh] will not take this into a private or a personal campaign of vindictiveness against a group that he disagreed with and accordingly, left," Mahdi says.
In voter surveys so far, Aboul Fotouh is far ahead of the Brotherhood's Morsi. Both are trailing Moussa, the former Arab League secretary general.
But Morsi could surge ahead when elections begin later this month if the Brotherhood is as successful at getting out the vote as it was during the parliamentary elections.