Agnes Lokudu, the head of the National Congress Party in south Sudan, blamed Thursday's shooting in Western Bahr al-Ghazal state on the region's local military.
"Three days ago at night some southern army soldiers came to the home of the president of the National Congress Party (NCP) in Raja, and killed him and eight other members of the NCP," Lokuda said.
Anger at NCP
The NCP, which dominates the north of the country, rules alongside the southern-based Sudan People's Liberation Movement (SPLM) as part of a peace deal that ended civil war in 2005, but there are significant tensions between the two parties.
Lokudu said the killings were motivated by anger that many people in the area had voted for the NCP.
Parties and independents opposing the SPLM, which leads the semi-autonomous government in the south, have previously complained of arrests and harassment.
But the SPLM denied that the separate south Sudan army was involved in the incident.
"This was a passionate crime to do with a wife - a feud that led to a shooting between the husband and lover," Suzanne Jambo, the head of the SPLM's external relations office, said.
"This is not political."
There has been little violence during the elections, despite tension being increased by boycotts of various parts of the process by opposition parties amid allegations of irregularities.
The SPLM pulled out of parliamentary voting in most northern states and withdrew Yasser Arman as a candidate for the presidency. The Umma party also pulled its candidate out the presidential race.
Those decisions left little competition for Omar Hassan al-Bashir, the current president and leader of the NCP who has in power for 22 years.
Ruling party offer
Al-Bashir's supporters seemed to reach out to the opposition parties on Wednesday, saying the NCP would invite opposition groupsto join the government if it won the current elections.
"If we are declared winners in the elections ... we would extend the invitation to all parties, even those who have not participated in the elections, to join the government," Ghazi Salaheddin, a senior leader of the NCP, said.
Al-Bashir's rivals gave a mixed reaction to the offer, which did not specify what role they might play in a new government.
"Let us talk about dialogue first, how to solve Sudan's problems," Fadlalla Burma Nasir, the vice-president of the opposition Umma party, said.
The SPLM's Arman said he was not interested in the offer and levelled further accusations of fraud against the ruling party.
"This is proof that they know the results in advance," he said.
Results of the elections are not expected until April 20.