These are the first national multi-party elections in 24 years
The US envoy to Sudan has held crisis talks in Khartoum after the decision of the main opposition candidate to pull out of April's presidential election.
US officials said Scott Gration would shuttle between meetings with leading government and opposition figures.
On Wednesday, the Sudan People's Liberation Movement (SPLM) said Yassir Arman would not stand because they felt the vote would not be free and fair.
Other opposition groups condemned the SPLM decision, calling it a "betrayal".
By James Copnall
BBC News, Khartoum
The SPLM's decision to withdraw Yassir Arman from the presidential race at first glance seems to seriously damage the poll's credibility. The blow will amplify in force if many or all of the northern opposition candidates pull out of the race too.
But President Omar al-Bashir may not be that disappointed.
The SPLM will contest the legislative elections everywhere except Darfur, which will allow the president's National Congress Party to say it has taken part in multi-party elections.
Mr Arman was one of - perhaps the - greatest threats to President Bashir in the presidential race. That has fuelled speculation, as yet unconfirmed, that some sort of deal was struck between the NCP and the SPLM.
Veteran Islamist leader Hassan al-Turabi confirmed that the candidates from his Popular Congress Party would not withdraw from the contest.
Other opposition parties were due to meet later to discuss whether to all boycott the vote - the first multi-party presidential election in 24 years.
Both the opposition parties and the SPLM - who serve in a coalition at national level with President Omar al-Bashir - believe the electoral process has been rigged in favour of his National Congress Party.
They say the registration process has been flawed, and their access to state media and rights to hold rallies restricted.
Explaining the withdrawal of his candidacy for the presidency on Wednesday, Mr Arman also said the ongoing conflict and state of emergency in the western region of Darfur made it "impossible to have free and fair elections".
"The people of Darfur in the internally displaced people's camps asked the SPLM not to be involved in the election," he said. "Our response to the people of Darfur's Political Bureau is that we have decided not to run."
The SPLM would, however, contest the 11-13 April parliamentary and municipal elections elsewhere in Sudan, Mr Arman added.
HIGH STAKES POLL
Africa's biggest country
Deeply divided along religious and ethnic lines
First multi-party national poll for 24 years
Continuing conflict in Darfur
President Bashir wanted for war crimes in Darfur
South Sudan rebuilding after 21 years of civil war
South Sudan could secede in 2011
Large oil fields near north-south border
President Bashir has threatened to cancel a promised referendum on independence for the South if the SPLM boycotts the poll.
The US, UK and Norway issued a joint statement saying they were "deeply concerned by reports of continued administrative and logistical challenges, as well as restrictions on political reforms".
But they warned that "irrespective of the outcome of elections", it was essential the January 2011 referendum go ahead as scheduled.
The head of the SPLM, Vice-President Salva Kiir, told the BBC there was no connection between the elections and the referendum.
"Just read the constitution," he said. "You will find that the referendum is something agreed alone and has to be conducted whether there are elections in Sudan or not," he said.
The SPLM joined the unity government in 2005 as part of a peace deal ending a two-decade civil war. Some 1.5 million people died in the conflict between the mainly Muslim North and the South, where most people are Christian or follow traditional beliefs.