CAIRO—Egypt's Muslim Brotherhood's agreement to back the secular, liberal opposition leader Mohamed ElBaradei as lead spokesman for the country's opposition groups in reform negotiations suggests the group's once sidelined moderate wing is regaining strength.

The move marks the latest step by the controversial Islamic organization to subordinate its religious goals to what opposition groups are describing as a battle for democracy, in a country run under a state of emergency by President Hosni Mubarak for more than 30 years. It also suggests the movement may be positioning itself as a significant political actor in future Egyptian politics.

Mr. ElBaradei, 68 years old, is the former head of the International Atomic Energy Agency, who won the Nobel Peace Prize for the group's nonproliferation work. He returned to Cairo on Thursday from Vienna after a long absence, to join protesters and to offer to lead a transitional government.

Early last year, the longtime activist played a major role in re-energizing Egypt's opposition movement, helping to organize a signature drive for a petition demanding political reform, although he didn't play a role in organizing the latest protests. He had clashed with the Bush administration over Iraq and Iran policies, but later forged ties with the Obama administration.

The Brotherhood, founded in 1928, is thought to be Egypt's most popular unofficial political organization. It has a long fought to establish Sharia law in Egypt, an anathema to the military leaders that have run the country and a key reason given by Mr. Mubarak and his predecessors for soft-pedaling on political reform. The group's strict views on morality and religion also have traditionally alienated them from Egypt's other political movements, which are largely led by Western-leaning intellectuals.

Photos: Sunday Protests Regional Upheaval A succession of rallies and demonstrations, in Egypt, Jordan, Yemen and Algeria have been inspired directly by the popular outpouring of anger that toppled Tunisian President Zine al-Abidine Ben Ali. See how these uprising progressed.

One of the hallmarks of the massive national protests against Mr. Mubarak has been its secular tone. Supporters of the Brotherhood have joined the street demonstrations, but their footprint has been intentionally light, according to opposition leaders. Brotherhood members agreed with the umbrella of opposition groups organizing the protests to keep religious slogans out of the demonstration to minimize the risk that Mr. Mubarak's security agencies could discredit the movement, organizers said.

Egypt's opposition groups have had a checkered past, with ideological divides and personal animosities sapping their strength against the might of the Mubarak regime. For now, their solidarity appears to be sticking.

The umbrella organization, called the National Association for Change, on Sunday formed a steering committee, with Mr. ElBaradei at the helm, to strategize further movements and pressure Mr. Mubarak and his military leaders for more political concessions, according to senior Brotherhood leaders.

That reflects the organization's strategy that their religious goals need to be put on the back burner to achieve democracy, said Helmi Gazzar, the head of the Brotherhood's district party office in northern Cairo.

"What we want is what the people want; right now we should have a completely different regime. We should have freedom and free elections," he said. "We respect Mr. Baradei. He has the most potential" to achieve this.

Some Middle East analysts argue that Egypt's Muslim Brotherhood, the organization from which many of the region's Islamists have taken inspiration, has a more moderate theological profile than is sometimes feared.

Unlike the Palestinian political group Hamas in the neighboring Gaza Strip, the Muslim Brotherhood isn't considered a terrorist organization by Washington or by European capitals. Egypt has outlawed the group as a political party, but members of the movement sit in Parliament as independent lawmakers, and U.S. officials frequently meet with these parliamentarians.

Detractors, however, see the Brotherhood as an extremist organization, similar to the Islamic movement that overthrew the Shah of Iran in 1979. They point to a draft political manifesto published by the organization in 2007 in which the organization called for a religious guidance counsel to be set up in Egypt to approve all laws passed by the country's civilian institutions. The political platform also states that Christians or women couldn't become president.

The Brotherhood's moderate wing disagreed with the manifesto, but the document helped exacerbate rifts between the group and Egypt's leftist and liberal democracy activists. For that reason, it was seen as a significant development when Mr. ElBaradei forged the umbrella opposition movement last year with the approval and inclusion of the Brotherhood.

Not all of the group's secular detractors have lost their mistrust of the Brotherhood, but they say they understand the importance of the group to the overall goal of pushing for democratic change.

"I have some fears about the Muslim Brotherhood and their [future] intentions. But the situation is bigger than all of us now. You need them in the streets," said Ziad el-Alami, a senior aide for Mr. ElBaradei and a human-rights lawyer.

—Charles Levinson contributed to this article.

Write to Margaret Coker at margaret.coker@wsj.com and Summer Said at summer.said@dowjones.com