Analysts say a possible "nightmare scenario" is a narrow government victory, aided by rigging.
"Then you are going to see the Raila camp go absolutely ballistic, because they've led polls," a Western diplomat said. He predicted such an outcome would produce rioting in Nairobi and Odinga's western homeland Nyanza.
ECONOMIC CREDENTIALS
Kibaki, 76, elected a legislator for every term since independence, hopes economic record of average 5 percent annual growth will win him a second term, before then retiring to his highland farm.
He has the support of his economically powerful Kikuyu tribe, the largest of Kenya's 40 or so ethnic groups, and has tempered his aloof image with skilful grassroots campaigning - in Mandela-like flowery shirts - in recent weeks.
Odinga has the ardent backing of his western Luo community, but has also garnered support from other ethnic groups who think the Kikuyus have had it too good under Kibaki.
Dubbed the "kingmaker" for helping Kibaki's 2002 win, before splitting with him over a referendum in 2005, Odinga belongs to one of Kenya's elite political dynasties. His father is left-leaning independence hero Jaramogi Oginga Odinga.
Both men are pledging broadly similar policies of increased economic growth along free-market lines and an extension of free education to include secondary schools.
Their pledges to outlaw corruption, however, have fallen on sceptical ears given that Kibaki's government has failed to tackle graft, while Odinga's team includes some people implicated by media and campaigners in scandals.
"The elite will always look after themselves," said Geoffrey Odindo, a street-cleaner on 3,500 shillings (28 pounds) a month.
Some 14 million of Kenya's 36 million people are eligible to vote from 6 a.m. (3:00 a.m. British time) to 6 p.m. (3:00 p.m. British time) on Thursday.
Official results are expected to start coming in from Friday morning, though media surveys will give a picture overnight.
