KHARTOUM, Sudan, Sept. 10 (UPI) -- A former southern Sudanese rebel group has signed an agreement with a northern opposition party to form an electoral alliance, the groups say.

The alliance between the Sudanese People's Liberation Movement of the semi-autonomous southern half of the country and the Islamic centrist Umma Party of former Sudanese Prime Minster Sadiq al-Mahdi presents the strongest challenge yet to President Omar al-Bashir's rule, The Christian Science Monitor reported.

The agreement to advance common goals and secure common interests comes seven months ahead of Sudan's general elections, scheduled for April.

The elections are seen as a potentially crucial step for peace in the war-torn North African country -- at war with itself for almost its entire post-colonial history, starting in 1956 -- since they would give Sudanese voters from north and south and even the strife-torn Darfur region their first chance to elect their leaders in a decade, the Monitor said.

"The SPLM is attempting to create a coalition that is a political counterweight to (Bashir's National Congress Party's) domination of the current system," John Prendergast, an Africa expert at the Washington-based Enough Project, a human rights group that focuses on the prevention of genocide, told the Monitor.

The National Congress has strong advantages over opposition parties, Prendergast said, and its past use of bribery and armed militias to undermine its enemies could be a strong motivation for the SPLM to use all methods, including peaceful political ones, to make friends.

With this alliance, "The NCP will have to rig the elections fairly profoundly in order to win next year's vote," he said, the Monitor reported.