Orange Democratic Movement Executive Director Oduor Ong’wen has been admitted to the Nairobi Hospital.
The Orange Party relayed the information to their supporters on Twitter.
They also re-assured the members of the public that Mr Ong'wen is recuperating as he is responding well to medication.
However, the party did not disclose the cause of his illness.
ODM Party Secretary-General Dr Agnes Zani was among senior party officials, family and friends who visited Ong'wen earlier on Thursday afternoon.
Ong'wen is among the leaders who are steering the People's Assembly committee which is planning the swearing in of NASA leaders Raila Odinga and Kalonzo Musyoka.
Other members of the team include Economist David Ndii, Peter Mathuki, Hamida Kibwana, Judy Sijeny, Dr Mutakha Kangu and Koitamet Ole Kina.
The team is also charged with the responsibility of steering the coalition's People's Assemblies across the country.
During the launch of the western region People's Assembly rally in Kakamega on Sunday, the NASA leader declared that he will be sworn in on January 30 at Uhuru Park.
[caption caption="NASA strategy committee members Koitamet ole Kina (left), Oduor Ong'wen (centre) and David Ndii"]
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The move is likely to be slowed down following the disagreements between ODM and the Wiper Party over the parliamentary committee leadership slots' allocation.
