Egypt’s most organized and powerful Islamist movement on Monday announced it intends to join forces with one of the nation’s oldest liberal parties, presenting a formidable coalition for upcoming parliamentary elections.

The alliance would unite the Muslim Brotherhood’s Freedom and Justice Party and the Wafd Party, a liberal party established just after World War I, to run on one candidate list in the elections scheduled for September. The move is likely to be a significant boost for the Brotherhood, a well-organized political group that expects to take a third of parliament’s seats.

The move surprised some analysts because of the long-standing rivalry between the two groups, and likely startled some secularists and liberals.

“Now that this coalition exists, it will dictate the electoral outcome,” said Essam el-Erian, vice president of the Freedom and Justice Party and former member of the Brotherhood’s guidance bureau. Erian said that the coalition is open and encourages others to join.

“We want a parliament that represents the entire nation, with all its political tendencies and forces,” he said.

The coalition of the two parties is the start of what Erian said the parties hope will be a far-reaching alliance with other groups and independent candidates. Ultimately, he said, they hope the coalition, after others join, will have a strong showing in the first parliament to be elected following President Hosni Mubarak’s ouster in February.

No single party is expected to win an outright majority in parliament, a body that will be tasked with choosing a 100-member committee to write Egypt’s new constitution.

While secular and liberal organizations are scrambling to form parties and campaign, the Brotherhood is reaching out to strengthen its existing base and eliminate competition with groups that have overlapping territory.

“This may give them a very solid base in areas they hadn’t previously been strong,” said Elijah Zarwan, an Egypt researcher at the International Crisis Group. The Brotherhood “is taking a long view of Egyptian politics. It’s a very significant switch.”

The Brotherhood, founded in 1928, first fielded “independent” candidates in 1984 in elections widely believed to be rigged in favor of Mubarak’s National Democratic Party. Despite being legally banned under Mubarak, it was allowed to exist under strict constraints and was considered one of the only alternatives to the ruling party.

The new alliance is one of “political opportunism,” said Nabil Fahmy, former Egyptian ambassador to the U.S. and dean of the school of public affairs at the American University of Cairo.

The announcement came a week after a group of liberal parties met and promised not to compete against each other. Fahmy said he was surprised the new coalition was formed so early when it’s still unclear if the elections will actually be held in September or be postponed, as many advocate.

“The Wafd gains the support from the Muslim Brotherhood and maybe the Muslim Brotherhood can gain from the Wafd’s image as a liberal party,” he said.

Also Monday, the military council agreed in a meeting with activists to look into cases of civilians convicted in military tribunals after the January uprising. Activists and lawyers say the tribunals have convicted thousands of civilians on trumped up charges.

Special correspondent Ingy Hassieb contributed to this report.