The Brotherhood’s entry into the race also turns the election into a debate over the future of the Islamist political movement that is sure to resonate in the region. Mr. Shater faces Islamist rivals to his left and right — one a more liberal former Brotherhood leader, the other an ultraconservative Salafist. Indeed, the Brotherhood may have entered the race in part because a strong showing by either rival could undercut the group’s authority as the predominant voice of Islam in Egyptian politics.

Mr. Shater is considered a conservative but a pragmatist. He has argued that Islam demands tolerance and democracy, has championed free trade and open markets and has guided the Brotherhood through its first public commitment to uphold the peace agreement with Israel.

But he also argues for an explicitly Islamic government. And while some in the group have argued that it should tolerate diverse approaches to Islamist politics from its own members, he has helped enforce the authority of the Brotherhood’s executive committee over its members, stirring allegations from liberals that it is undemocratic.

Doubts about the strength of the Brotherhood’s commitment to its promises raise particular concerns in the United States and Israel, which considered the Mubarak government’s commitment to the peace agreement a linchpin of regional stability.

An Israeli official, speaking on condition of anonymity, declined to comment specifically on Mr. Shater, but called the nomination worrisome. “Obviously this is not good news,” the official said. “The Muslim Brotherhood is no friend of Israel’s. They do not wish us well.” The official added: “The big question will be how pragmatic they will be once in power. It could go in either direction.”