The Umma party and the Sudan People's Liberation Movement (SPLM) have both withdrawn from the presidential election.
The SPLM, which is the ruling party in the semi-autonomous south, will however compete for parliamentary seats in the southern states and the sensitive Blue Nile and south Kordofan states.
"When we reviewed what had happened, we found that the elections, even if it was popular, the results would not be good," Salva Kiir, the president of South Sudan, said as he campaigned on Friday.
A group of parliamentarians from 12 other countries have called for the elections to be closely monitored for irregularities.
"We are deeply concerned about the inadequate level of preparation for the upcoming elections in Sudan and the threat of violence and intimidation to civil society," they wrote in an open letter.
"The building blocks of a free and fair election... are not in place," the deputies from Australia, Afghanistan, Bahrain, Britain, Egypt, France, Germany, Italy, Kuwait, Mauritania, Morocco and Yemen said.
Jimmy Carter, the former US president, expressed regret at the decision of some political parties to boycott the polls as he arrived in Sudan.
"We are glad that everything at this point is peaceful and many parties are participating, so we look forward to successful elections," Carter said after arriving in Sudan on Thursday.
"We are hoping that it will be a fair and honest election, at least for the ones who are participating ... There are about 16,000 candidates still involved in the election."
The Carter Centre, the only international long-term observer mission in Sudan, said last month that a short delay to the elections may be necessary because of logistical problems.
Bashir threatened last month to expel election observers after the Carter Centre called for the delay.
On Friday, the information ministry of Bashir's National Congress Party (NCP) dismissed the effect of the withdrawals on the elections.
"I think that these accusations and allegations from different oppositions, political parties, this is due because they discover that they are very weak and not in a position to compete with the National Congress Party," an advisor told the Reuters news agency.
"That is why they come with these accusations, so that the election will not be carried out or at least they can create some confusion to the public, but I don't think up until now there is any negative effect on the people."
Analysts say that Bashir wants the elections to legitimise his rule, in defiance of an International Criminal Court warrant for his arrest for war crimes in the western Darfur region.
On Wednesday, an international observer mission from the European Union said that it would withdraw its observers from the Darfur region because violence and kidnappings were hindering their work.
The UN estimates that about 300,000 died in the humanitarian crisis and more than 2.5 million fled their homes during the Darfur conflict.
Bashir's NCP has ruled Sudan in a coalition with the SPLM since a peace deal that ended war between the two sides of Sudan in 2005.