The Muslim Brotherhood’s political party said Thursday night that its candidate was leading in Egypt’s first free presidential race, a sharp illustration of the strength of the Islamist group’s political machine.

A victory in the first round of voting for Mohammed Morsi would set the stage for a polarizing runoff next month. Although ballots were still being counted early Friday, Morsi could face a moderate Islamist who broke ranks with the Brotherhood or one of two contenders who served in key roles under President Hosni Mubarak.

If Morsi were to become president, the long-outlawed Brotherhood would have a near-monopoly on political power in the country, making it the runaway victor in a popular revolt it was initially reluctant to embrace. But prevailing in a second round could be challenging for Morsi, analysts say, because most potential rivals would probably appeal to a broad set of constituencies that do not want the Islamist group to run Egypt.

The Egyptian transition could have far-reaching implications for U.S. interests in the region, chief among them Cairo’s peace treaty with Israel and the regional stability it represents. In recent months, U.S. officials have sought assurances from the Brotherhood and other emerging political groups that continued American military aid will ensure the treaty is honored.

For the United States and Egypt, analysts said, one advantage of a strong Morsi showing could be short-term stability.

“Because of how organized the Brotherhood are, Morsi is a result that’s not likely to provoke questions about the legitimacy of the election,” said Michele Dunne, a former Middle East specialist at the National Security Council who is now at the Atlantic Council.

“On the other hand, an Islamist president is going to come in with potentially a very different view of world affairs. No one knows how they will handle the crises that will inevitably occur, whether it’s Israel, the West Bank, Gaza or Lebanon.”

Brotherhood leaders said at a news conference late Thursday that preliminary counts from 236 of the 13,000 polling stations suggested that Morsi is leading with more than 40 percent of the vote.

“The Egyptian people proved that they are able to form a new regime,” said Essam el-Erian, a senior Brotherhood leader. “We respect the results whatever the outcome is.”

Officials results are scheduled to be released Tuesday. With none of the 13 candidates expected to garner 50 percent of the vote, a runoff between the top two vote-getters is scheduled for June 16-17. The presidential election commission told reporters Thursday night that slightly more than 40 percent of Egypt’s 52 million registered voters had cast ballots in the first round.

Unexpected surge

The strong support over the past two days for Morsi — an uncharismatic politician who was unknown to most Egyptians until recently — is the latest sign that conservative Muslim political parties are gaining the most from the regional revolutions that erupted early last year, toppling four autocratic rulers and upending the geopolitics of the Middle East and North Africa.

“The Brotherhood still has an unparalleled electoral machine,” said Shadi Hamid, an Egypt expert at the Brookings Doha Center who traveled to Cairo to witness the vote. “I’ve been impressed by how quickly the Brotherhood has been able to gain ground.”

The campaigns of several other front-runners made competing claims Thursday night, each saying their candidate would face off against Morsi.

Until recently, Morsi’s campaign had been regarded as a lost cause. But Thursday, aides went as far as to predict that he could avoid a runoff, suggesting that he may have won more than half of the votes cast — a prospect analysts considered highly unlikely.

A prominent Brotherhood member who agreed to speak only on the condition of anonymity said Morsi’s strong showing was largely the result of an “amazing network” of parliamentarians who campaigned for him nationwide. The Brotherhood won just under half of the seats in parliament in elections in November.

The campaign of Amr Moussa, who once served as foreign minister under Mubarak, projected that Morsi was ahead with about 25 percent of the vote, trailed by Moussa with 23 percent, based on exit polls.

“The turnout is not very good today,” Moussa spokesman Ahmed Kamel said Thursday, the second and final day of voting. “His voters show up anyway. They bus them in.”

Moussa, a secular candidate, is a known quantity to international leaders from his time as chief of the Arab League. He presents one of the more reassuring options for the United States, said Marina Ottaway, an Egypt expert at the Carnegie Endowment for International Peace. Although he has often made negative comments about Israel, “he certainly wouldn’t abrogate the peace treaty or do something stupid on that front,” she said.

The other front-runners are Abdel Moneim Aboul Fotouh, a former leader of the Brotherhood who split with the party to launch his own presidential bid; Ahmed Shafiq, Mubarak’s last prime minister; and Hamdeen Sabahi, an Arab nationalist who is the only secular candidate without ties to Mubarak’s government.

Brotherhood’s sway

The Muslim Brotherhood, which had long been barred from forming a political party, launched its Freedom and Justice Party shortly after Mubarak stepped down Feb. 11, 2011. Its leaders initially said that the party would compete for no more than a third of the seats in parliament and that it would sit out the first post-revolution presidential race. But they backtracked on both promises, arguing that the group needed to flex its political muscles to ensure that the military chiefs who took the reins of the country after Mubarak’s ouster made good on their promise to oversee a genuine transition to democratic rule.

The presidential candidate the group backed initially, Khairat el-Shater, is a millionaire who has been the Brotherhood’s chief strategist. But he was barred from running a month ago because he had been a political prisoner, forcing the Brotherhood to quickly thrust Morsi into the spotlight.

“Mohammed Morsi is the only one of the Egyptian candidates that people aren’t voting for because of his personality but because of the Brotherhood,” the prominent Brotherhood member said.

That view was backed by interviews with voters who support Morsi.

Hussein Sadiq Mohammed, a 48-year-old lawyer who voted for Morsi, said he knew little about the candidate but felt confident knowing that Morsi, an engineer, was educated in Egypt and the United States. More significant for him, Sadiq said, was the values the party represents. Pulling out a folded Morsi flier from his dress shirt, the lawyer pointed to the scale, a symbol the campaign has used to illustrate its commitment to justice.

“I feel he will unite the country,” Mohammed said.

Staff writer William Wan in Washington contributed to this report.