Though official final results are to be released in a few days, early returns show that about 20 percent voted for Abdel Moneim Aboul Fotouh, a former Brotherhood leader campaigning as both an Islamist and as a liberal in an effort to break out of Egypt’s culture war. And another roughly 20 percent voted for Hamdeen Sabahi, a secular populist with a record of fighting the Mubarak government on behalf of the poor. (Fifth place went to Amr Moussa, a former foreign minister who presented a softer and more conciliatory version of Mr. Shafik’s secular law-and-order appeal.)

Handicapping the runoff was all but impossible. Although candidates from the Muslim Brotherhood or more conservative Islamist parties won about three-quarters of the seats in Parliament, Islamists and more secular candidates split the vote in the first stage of the presidential election. It was unclear whether voters who picked Mr. Sabahi, the secular populist, would lean toward Mr. Morsi to avoid returning a Mubarak minister to power, or to Mr. Shafik in order to avoid giving so much power to the Brotherhood. But at the end of the day, the possibility of low turnout favors the Brotherhood because its vast political machine can drive its voters to the polls.

As soon as the results became clear, each of the two leading candidates began to try to shift to the center, by rallying against the other. In a Friday night news conference, officials of the Muslim Brotherhood announced that they were inviting the other “revolutionary candidates” — effectively, all but Mr. Shafik — to a meeting to talk about a coalition to oppose the former prime minister and about sharing power in a Brotherhood-led government.

“Rescuing the homeland includes securing victory for a candidate who belongs to the revolutionary camp, and the camp that struggles against the old regime,” Essam el-Erian, a Brotherhood lawmaker, said, trying to portray Mr. Morsi as the champion of the whole popular uprising and not just the Islamist forces.

Supporters of Mr. Shafik, meanwhile, circulated a cellphone message urging unity against the Brotherhood. “I beg you to please put your differences aside and go vote for Shafik not because you believe in him but because it will be a catastrophe if we consolidate all power to one party (presidency and Parliament)!” the message read. “History has proved, so please spread!”