Candidates from Sudan's four main opposition parties have withdrawn from the country's upcoming presidential election.
Representatives from the Umma Party announced the decision Thursday, a day after the the SPLM, the ruling party of semi-autonomous southern Sudan, also pulled out of the presidential race.
In an interview Thursday with VOA's English to Africa service, SPLM candidate Yasir Arman said he is boycotting the poll because of vote rigging by the National Congress Party of President Omar al-Bashir.
He also said his decision is based on the conflict in Sudan's Darfur region.
Arman was seen as one of the favorites to challenge Mr. Bashir in voting that begins April 11.
The U.S. envoy to Sudan, Scott Gration, is meeting with political leaders in Khartoum in an effort to save the credibility of the elections.
The SPLM and NCP are part of a fragile coalition government set up under the 2005 peace deal that ended Sudan's north-south civil war. The SPLM says it will still contest regional and legislative elections across Sudan, but not in Darfur.
On Tuesday, the International Crisis Group also accused the NCP of trying to rig the elections.
The conflict prevention group says the NCP has drafted election laws in its favor. It also says President Bashir's party has "bought tribal loyalties" and used flawed census results to draft electoral districts.
The Brussels-based group was especially critical of the electoral process in Darfur. It said the ruling party went to great lengths to count its supporters in Darfur in the 2008 census but made little effort to include the nearly 2.6 million people who have been internally displaced by the region's ongoing conflict.
The Darfuri vote is critical because nearly 20 percent of Sudan's citizens live in the region. Darfur also holds 86 of the 450 seats in the national assembly.
The vote will be Sudan's first multi-party elections since 1986 and it is a key part of a 2005 peace deal that ended the country's north-south civil war.
On Monday, President Bashir said if former southern rebels boycott the elections, the Sudanese government will not allow the south to hold a planned referendum on becoming independent.