Kipkalya Kones, 56, was planning to campaign in by-elections
Kenya is holding by-elections in five constituencies, less then six months after the country was gripped by violence following disputed polls.
Two of the seats in question were held by MPs killed after December's polls.
But the voting will be overshadowed by the deaths of two government ministers in a plane crash on Tuesday.
Roads Minister Kipkalya Kones and Assistant Home Affairs Minister Lorna Laboso were on their way to assist with the by-elections.
A pilot and a security guard were also killed when the Cessna plane the ministers were flying in crashed near the western town of Narok, Kenyan police told the BBC.
Balance of power
The BBC's Kevin Mwachiro in Embakasi says there was a low turn-out in the morning, although by-elections typically do not attract high numbers of voters.
He says Electoral Commission of Kenya officials were hopeful that turn-out would improve later.
There are fears that the underlying tensions that sparked clashes after the polls have still not been resolved and could resurface, the BBC's Karen Allen in Nairobi says.
Kenya's grand coalition government - which has set up a number of commissions to investigate the violence - has been looking decidedly fragile, our correspondent says.
The results could also upset the delicate balance of power in parliament.
Should the Orange Democratic Movement lose its majority in parliament, party leader Raila Odinga's position as prime minister in the coalition government could be uncertain.
The ODM will hope to retain the seats of Embakasi in Nairobi, as well as Ainamoi and Emuhaya in the Rift Valley, scene of the worst violence earlier this year.
But President Mwai Kibaki's Party of National Unity is putting up a spirited fight.
"The numbers are very tight in terms of who controls parliament. With the death of the minister and the assistant minister now the ODM has 100 MPs and the PNU coalition has 102 MPs," says analyst Kwamchetsi Makokha.
There have also been reports of hate leaflets being circulated in the Kilgoris constituency, raising the spectre of ethnic violence which was blamed for some of the post-poll violence.
Supporters of President Kibaki and Mr Odinga have locked horns over several key areas, including whether those held after the elections should be given amnesty or be subject to the full force of the law.
More than 1,000 people were killed and some 300,000 displaced after the polls.