Nazarbayev, who has been in power for 16 years, has pledged that Sunday's vote will be free and fair.
But his main challenger Zharmakhan Tuyakbai, who is running on behalf of a pro-democracy opposition alliance, says he expects foul play.
"I am sure the vote will be fraudulent," Tuyakbai said.
Diplomats share his scepticism, given that the run-up to the vote saw one major party banned outright, seizures by police of pro-opposition newspapers and leaflets, and media bias in favour of Nazarbayev.
The country has never held an election judged free and fair by Western observers.
Last year, Kazakhstan held a parliamentary election that gave the opposition just one seat in the lower house.
In protest, the politician who won it declined to take it up.
"I am sure the vote will be fraudulent"
The stability and oil wealth of Kazakhstan have kept Nazarbayev popular among many for the 16 years he has ruled, despite pockets of extreme rural poverty, corruption scandals and a record of votes that observers say are getting more flawed rather than less.
Nazarbayev is said to be the man to offer the best bet for the West's long-term policy interests: oil production and regional stability.
Opposition candidate Zamakhan
Tuyakbai expects voting fraud
One Western diplomat in the biggest city, Almaty, said the vote for a new seven-year presidential term would be a test of whether former Soviet states could hold untroubled elections at all after revolutions in Georgia, Ukraine and Kyrgyzstan and protests in Azerbaijan.
The United States says its support of democracy is clear.
"These elections are important, because they will be a demonstration of whether and how Kazakhstan is prepared to carry out its commitments to modernise and democratise the country," US ambassador John Ordway said.
The main opposition grouping, For a Just Kazakhstan, however, blame the West for putting oil before democracy.
Tuyakbai said he was disappointed by what he described as lukewarm Western support for democratic reforms in the oil-rich country, where no election has met international standards.
European countries and the United States, which have invested heavily in Kazakhstan's energy industry, had opted instead to protect their financial and strategic goals, he said.
President Nazarbayev is widely
expected to be declared winner
"Development of democracy could be the victim of economic interests," he said.
Tuyakbai added that he believed the West applied double standards by demanding democratic elections in Ukraine and Belarus in the European part of the former Soviet Union, but not doing so in Central Asia.
The Kazakh leader, 65, has been in power since 1989, when he held the top Communist Party post in what was then Soviet Kazakhstan.
He is expected to win easily on Sunday. He was first elected president in an uncontested vote in 1991, shortly before the country became independent.
In 1995, his mandate and his powers were extended through a referendum and in 1999 he won early elections.
He stresses his success in maintaining stability and pulling the country from post-Soviet economic ruin.
Generally seen as the most powerful opposition candidate, Tuyakbai, 58, was once a Nazarbayev ally before breaking ranks from the president's Otan party last year.
Tuyakbai won the backing of several opposition groups who joined in a coalition called For a Just Kazakhstan.
He has focused on allegations of corruption under Nazarbayev and also vows to fight poverty.
Baimenov, 46, is another ex-ally of Nazarbayev, having served as labour minister in 2000-2001 and previous to that as head of the presidential administration.
He is the only member of the opposition to have won a seat in parliament in 2004, but he boycotted the legislature in protest at the way the election was held.
A longtime environmental activist, Eleusizov, 55, heads the Tabighat (Nature) movement and is a veteran of campaigns against nuclear testing in Soviet Kazakhstan, as well as a member of a committee to save the Aral Sea.
He has never run for office and describes himself in this race as an independent.
Abylkasymov, 57, represents the Communist People's Party of Kazakhstan, which broke off from the Communist Party.
He favours a return to Communist rule.